diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 04b0d7ebef9..4cc9fe72bb0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -75,7 +75,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        ocamlv: [ 4.02.3, 4.03.0, 4.04.2, 4.05.0, 4.06.1, 4.07.1, 4.08.1, 4.09.1, 4.10.2, 4.11.2, 4.12.0 ]
+        ocamlv: [ 4.03.0, 4.04.2, 4.05.0, 4.06.1, 4.07.1, 4.08.1, 4.09.1, 4.10.2, 4.11.2, 4.12.0 ]
         include:
           - os: macos-latest
             ocamlv: 4.12.0
diff --git a/configure b/configure
index 59d8c0e217a..73fb7f5fa78 100755
--- a/configure
+++ b/configure
@@ -2185,7 +2185,7 @@ if  test -x bootstrap/ocaml/bin/ocamlc -o -x bootstrap/ocaml/bin/ocamlopt ; then
 fi
 
 # XXX This isn't strictly correct for Windows
-MIN_OCAML_VERSION=4.02.3
+MIN_OCAML_VERSION=4.03.0
 
   # checking for ocamlc
   if test -n "$ac_tool_prefix"; then
@@ -3504,7 +3504,7 @@ CPATH=$CPATH
 LIBRARY_PATH=$LIBRARY_PATH
 
 
-# Check that OCaml version is greater or equal to 4.02.3
+# Check that OCaml version is greater or equal to 4.03.0
 # Native Windows builds require at least 4.06.0 for the Unicode runtime.
 for ac_prog in gawk mawk nawk awk
 do
@@ -3585,7 +3585,7 @@ x$ax_compare_version_B" | sed 's/^ *//' | sort -r | sed "s/x${ax_compare_version
 
 
     if test "$ax_compare_version" = "true" ; then
-    as_fn_error $? "Your version of OCaml: $OCAMLVERSION is not supported" "$LINENO" 5
+    as_fn_error $? "Your version of OCaml ($OCAMLVERSION) is not supported" "$LINENO" 5
       fi
 
 
diff --git a/configure.ac b/configure.ac
index b786aae4cc8..c87b5b297a5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -15,7 +15,7 @@ AS_IF([ test -x bootstrap/ocaml/bin/ocamlc -o -x bootstrap/ocaml/bin/ocamlopt ],
 ])
 
 # XXX This isn't strictly correct for Windows
-MIN_OCAML_VERSION=4.02.3
+MIN_OCAML_VERSION=4.03.0
 
 AC_PROG_OCAML
 if test "x$OCAMLC" = "xno"; then
@@ -86,13 +86,13 @@ AC_SUBST(LIB_PREFIX,$LIB_PREFIX)
 AC_SUBST(CPATH,$CPATH)
 AC_SUBST(LIBRARY_PATH,$LIBRARY_PATH)
 
-# Check that OCaml version is greater or equal to 4.02.3
+# Check that OCaml version is greater or equal to 4.03.0
 # Native Windows builds require at least 4.06.0 for the Unicode runtime.
 AS_IF([test "x${enable_version_check}" != "xno"], [
   AS_IF([ test ${WIN32} -eq 1 ],[MIN_OCAML_VERSION=4.06.0])
   AX_COMPARE_VERSION(
     [$OCAMLVERSION], [lt], [$MIN_OCAML_VERSION],
-    AC_MSG_ERROR([Your version of OCaml: $OCAMLVERSION is not supported]))
+    AC_MSG_ERROR([Your version of OCaml ($OCAMLVERSION) is not supported]))
 ])
 
 AC_MSG_CHECKING([for compiler type])
diff --git a/doc/modules b/doc/modules
index 4054065b115..bb6f19c9620 100644
--- a/doc/modules
+++ b/doc/modules
@@ -11,7 +11,7 @@ src
 │   ├── opamJson.ml                     Wrapper on Jsonm; only needed for some debug options
 │   ├── opamStd.ml                      Generic stdlib functions (String, List, Option, Sys submodules...)
 │   ├── opamConsole.ml                  Console output, ANSI color, logging and user querying
-│   ├── opamCompat.ml.4.01/4.02         Compatibility layer (Bytes, etc.) for different OCaml versions
+│   ├── opamCompat.ml                   Compatibility layer for different OCaml versions
 │   │   # system handling
 │   ├── opamProcess.ml                  Process and job handling, with logs, termination status, etc.
 │   ├── opamSystem.ml                   Bindings of lots of filesystem and system operations
diff --git a/master_changes.md b/master_changes.md
index 698af60023e..6b11787bb5d 100644
--- a/master_changes.md
+++ b/master_changes.md
@@ -77,6 +77,7 @@ Prefixes used to help generate release notes, changes, and blog posts:
 ## Build
   * Bump src_exts and fix build compat with Dune 2.9.0 [#4752 @dra27]
   * Upgrade to dose3 >= 6.1 and vendor dose3 7.0.0 [#4760 @kit-ty-kate]
+  * Change minimum required OCaml to 4.03.0 [#4770 @dra27]
 
 ## Infrastructure
   *
diff --git a/opam-client.opam b/opam-client.opam
index 45bfbee35d8..65492ea91e2 100644
--- a/opam-client.opam
+++ b/opam-client.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-state" {= version}
   "opam-solver" {= version}
   "base64" {>= "3.1.0"}
diff --git a/opam-core.opam b/opam-core.opam
index 35f04ab4a91..5f15ff3a5c5 100644
--- a/opam-core.opam
+++ b/opam-core.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "base-unix"
   "base-bigarray"
   "ocamlgraph"
diff --git a/opam-devel.opam b/opam-devel.opam
index 9ee69c0e91f..94657675f3c 100644
--- a/opam-devel.opam
+++ b/opam-devel.opam
@@ -26,7 +26,7 @@ build: [
   [make "%{name}%.install"]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-client" {= version}
   "cmdliner" {>= "0.9.8"}
   "dune" {>= "1.11.0"}
diff --git a/opam-format.opam b/opam-format.opam
index eac89aee9ad..09ee763f6a2 100644
--- a/opam-format.opam
+++ b/opam-format.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-core" {= version}
   "opam-file-format" {>= "2.1.3"}
   "re" {>= "1.9.0"}
diff --git a/opam-installer.opam b/opam-installer.opam
index 90a7beb77a7..7be49878f2a 100644
--- a/opam-installer.opam
+++ b/opam-installer.opam
@@ -28,7 +28,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-format" {= version}
   "cmdliner" {>= "0.9.8"}
   "dune" {>= "1.11.0"}
diff --git a/opam-repository.opam b/opam-repository.opam
index 9f9f5e4656e..b1d4c7608fb 100644
--- a/opam-repository.opam
+++ b/opam-repository.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-format" {= version}
   "dune" {>= "1.11.0"}
 ]
diff --git a/opam-solver.opam b/opam-solver.opam
index e44dd609275..2d5a18b4294 100644
--- a/opam-solver.opam
+++ b/opam-solver.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-format" {= version}
   "mccs" {>= "1.1+9"}
   "dose3" {>= "6.1"}
diff --git a/opam-state.opam b/opam-state.opam
index 995985d3fed..4027953dc9e 100644
--- a/opam-state.opam
+++ b/opam-state.opam
@@ -26,7 +26,7 @@ build: [
   ["dune" "build" "-p" name "-j" jobs]
 ]
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.03.0"}
   "opam-repository" {= version}
   "re" {>= "1.9.0"}
   "dune" {>= "1.11.0"}
diff --git a/src/client/opamCLIVersion.ml b/src/client/opamCLIVersion.ml
index 728e1683613..8c941bfcc23 100644
--- a/src/client/opamCLIVersion.ml
+++ b/src/client/opamCLIVersion.ml
@@ -8,8 +8,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 type t = int * int
 
 let supported_versions = [(2, 0); (2, 1)]
@@ -41,9 +39,9 @@ let of_json = function
   | `String x -> of_string_opt x
   | _ -> None
 
-let ( >= ) = Stdlib.( >= )
-let ( < ) = Stdlib.( < )
-let compare = Stdlib.compare
+let ( >= ) = OpamCompat.Stdlib.( >= )
+let ( < ) = OpamCompat.Stdlib.( < )
+let compare = OpamCompat.Stdlib.compare
 
 let previous cli =
   let f previous version =
@@ -66,9 +64,9 @@ module Sourced = struct
 end
 
 module Op = struct
-  let ( @>= ) (c,_) = Stdlib.( >= ) c
-  let ( @< ) (c,_) = Stdlib.( < ) c
-  let ( @= ) (c,_) = Stdlib.( = ) c
+  let ( @>= ) (c,_) = OpamCompat.Stdlib.( >= ) c
+  let ( @< ) (c,_) = OpamCompat.Stdlib.( < ) c
+  let ( @= ) (c,_) = OpamCompat.Stdlib.( = ) c
 end
 
 module O = struct
diff --git a/src/client/opamCliMain.ml b/src/client/opamCliMain.ml
index 6d95049bae1..4a4c822f568 100644
--- a/src/client/opamCliMain.ml
+++ b/src/client/opamCliMain.ml
@@ -10,7 +10,6 @@
 (**************************************************************************)
 
 open Cmdliner
-open OpamTypes
 open OpamStateTypes
 open OpamTypesBase
 open OpamStd.Op
@@ -24,7 +23,7 @@ exception InvalidFlagContent of string * (string * string) option
 exception InvalidNewFlag of OpamCLIVersion.Sourced.t * string * OpamCLIVersion.t
 
 let raise_invalid_cli :
-  (OpamCLIVersion.Sourced.t, string option) OpamCompat.Result.t -> 'a
+  (OpamCLIVersion.Sourced.t, string option) result -> 'a
   = function
     | Ok ocli -> raise (InvalidCLI ocli)
     | Error None -> raise (InvalidFlagContent ("cli", None))
diff --git a/src/client/opamCommands.ml b/src/client/opamCommands.ml
index 7ebfd1aab45..e296f074d0f 100644
--- a/src/client/opamCommands.ml
+++ b/src/client/opamCommands.ml
@@ -13,7 +13,6 @@ open Cmdliner
 open OpamArg
 open OpamTypes
 open OpamStateTypes
-open OpamTypesBase
 open OpamStd.Op
 
 let self_upgrade_exe opamroot =
diff --git a/src/client/opamListCommand.ml b/src/client/opamListCommand.ml
index 0e30772c14f..9ce2ff2a031 100644
--- a/src/client/opamListCommand.ml
+++ b/src/client/opamListCommand.ml
@@ -9,7 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
 open OpamParserTypes.FullPos
 open OpamTypes
 open OpamStateTypes
diff --git a/src/core/opamCompat.ml b/src/core/opamCompat.ml
index f0f944b38e2..d3086e6e74b 100644
--- a/src/core/opamCompat.ml
+++ b/src/core/opamCompat.ml
@@ -8,30 +8,9 @@
 (*                                                                        *)
 (**************************************************************************)
 
-module String =
-#if OCAML_VERSION >= (4, 3, 0)
-  String
-#else
-struct
-  include String
+module String = String
 
-  let lowercase_ascii = lowercase
-  let uppercase_ascii = uppercase
-  let capitalize_ascii = capitalize
-end
-#endif
-
-module Char =
-#if OCAML_VERSION >= (4, 3, 0)
-  Char
-#else
-struct
-  include Char
-
-  let lowercase_ascii = lowercase
-  let uppercase_ascii = uppercase
-end
-#endif
+module Char = Char
 
 module Printexc =
 #if OCAML_VERSION >= (4, 5, 0)
@@ -55,17 +34,7 @@ struct
 end
 #endif
 
-module Uchar =
-#if OCAML_VERSION >= (4, 3, 0)
-  Uchar
-#else
-struct
-  type t = int
-
-  let of_int i = i
-  external to_int : t -> int = "%identity"
-end
-#endif
+module Uchar = Uchar
 
 module Buffer =
 #if OCAML_VERSION >= (4, 6, 0)
@@ -117,13 +86,13 @@ module Result =
 #else
 struct
   type ('a, 'e) t
-#if OCAML_VERSION >= (4, 3, 0)
     = ('a, 'e) result
-#endif
     = Ok of 'a | Error of 'e
 end
 #endif
 
 #if OCAML_VERSION < (4, 7, 0)
 module Stdlib = Pervasives
+#else
+module Stdlib = Stdlib
 #endif
diff --git a/src/core/opamCompat.mli b/src/core/opamCompat.mli
index ff1920fad8a..48c0b4ca999 100644
--- a/src/core/opamCompat.mli
+++ b/src/core/opamCompat.mli
@@ -8,30 +8,9 @@
 (*                                                                        *)
 (**************************************************************************)
 
-module String
-#if OCAML_VERSION >= (4, 3, 0)
-= String
-#else
-: sig
-  include module type of struct include String end
+module String = String
 
-  val lowercase_ascii : string -> string
-  val uppercase_ascii : string -> string
-  val capitalize_ascii : string -> string
-end
-#endif
-
-module Char
-#if OCAML_VERSION >= (4, 3, 0)
-= Char
-#else
-: sig
-  include module type of struct include Char end
-
-  val lowercase_ascii: char -> char
-  val uppercase_ascii: char -> char
-end
-#endif
+module Char = Char
 
 module Printexc
 #if OCAML_VERSION >= (4, 5, 0)
@@ -57,17 +36,7 @@ module Unix
 end
 #endif
 
-module Uchar
-#if OCAML_VERSION >= (4, 3, 0)
-= Uchar
-#else
-: sig
-  type t
-
-  val of_int : int -> t
-  external to_int : t -> int = "%identity"
-end
-#endif
+module Uchar = Uchar
 
 module Buffer
 #if OCAML_VERSION >= (4, 6, 0)
@@ -97,13 +66,13 @@ module Result
 #else
 : sig
   type ('a, 'e) t
-#if OCAML_VERSION >= (4, 3, 0)
     = ('a, 'e) result
-#endif
     = Ok of 'a | Error of 'e
 end
 #endif
 
 #if OCAML_VERSION < (4, 7, 0)
 module Stdlib = Pervasives
+#else
+module Stdlib = Stdlib
 #endif
diff --git a/src/core/opamConsole.ml b/src/core/opamConsole.ml
index 9098ea01459..c8eb45d2746 100644
--- a/src/core/opamConsole.ml
+++ b/src/core/opamConsole.ml
@@ -9,8 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 (* Global configuration *)
 
 let debug () = abs OpamCoreConfig.(!r.debug_level) > 0
@@ -165,7 +163,7 @@ let utf8_symbol main ?(alternates=[]) s =
           main
       in
       let b = Buffer.create 4 in
-      Buffer.add_utf_8_uchar b scalar;
+      OpamCompat.Buffer.add_utf_8_uchar b scalar;
       Buffer.contents b
     with Failure _
        | Not_found ->
diff --git a/src/core/opamConsole.mli b/src/core/opamConsole.mli
index be08f5d8dcb..39c8412d2c7 100644
--- a/src/core/opamConsole.mli
+++ b/src/core/opamConsole.mli
@@ -46,30 +46,30 @@ val acolor : text_style -> unit -> string -> string
 val acolor_w : int -> text_style -> Format.formatter -> string -> unit
 
 module Symbols : sig
-  val rightwards_arrow : OpamCompat.Uchar.t
-  val box_drawings_light_down_and_right : OpamCompat.Uchar.t
-  val box_drawings_light_horizontal : OpamCompat.Uchar.t
-  val box_drawings_light_vertical : OpamCompat.Uchar.t
-  val box_drawings_light_up_and_right : OpamCompat.Uchar.t
-  val box_drawings_light_right : OpamCompat.Uchar.t
-  val circled_division_slash : OpamCompat.Uchar.t
-  val asterisk_operator : OpamCompat.Uchar.t
-  val north_east_arrow : OpamCompat.Uchar.t
-  val south_east_arrow : OpamCompat.Uchar.t
-  val clockwise_open_circle_arrow : OpamCompat.Uchar.t
-  val greek_small_letter_lambda : OpamCompat.Uchar.t
-  val latin_capital_letter_o_with_stroke : OpamCompat.Uchar.t
-  val six_pointed_black_star : OpamCompat.Uchar.t
-  val upwards_arrow : OpamCompat.Uchar.t
-  val downwards_arrow : OpamCompat.Uchar.t
-  val up_down_arrow : OpamCompat.Uchar.t
-  val downwards_double_arrow : OpamCompat.Uchar.t
-  val downwards_black_arrow : OpamCompat.Uchar.t
-  val black_down_pointing_triangle : OpamCompat.Uchar.t
+  val rightwards_arrow : Uchar.t
+  val box_drawings_light_down_and_right : Uchar.t
+  val box_drawings_light_horizontal : Uchar.t
+  val box_drawings_light_vertical : Uchar.t
+  val box_drawings_light_up_and_right : Uchar.t
+  val box_drawings_light_right : Uchar.t
+  val circled_division_slash : Uchar.t
+  val asterisk_operator : Uchar.t
+  val north_east_arrow : Uchar.t
+  val south_east_arrow : Uchar.t
+  val clockwise_open_circle_arrow : Uchar.t
+  val greek_small_letter_lambda : Uchar.t
+  val latin_capital_letter_o_with_stroke : Uchar.t
+  val six_pointed_black_star : Uchar.t
+  val upwards_arrow : Uchar.t
+  val downwards_arrow : Uchar.t
+  val up_down_arrow : Uchar.t
+  val downwards_double_arrow : Uchar.t
+  val downwards_black_arrow : Uchar.t
+  val black_down_pointing_triangle : Uchar.t
 end
 
 val utf8_symbol:
-  OpamCompat.Uchar.t -> ?alternates:OpamCompat.Uchar.t list -> string -> string
+  Uchar.t -> ?alternates:Uchar.t list -> string -> string
 
 (** Logging *)
 
diff --git a/src/core/opamCoreConfig.ml b/src/core/opamCoreConfig.ml
index 53687d73104..065fe911d53 100644
--- a/src/core/opamCoreConfig.ml
+++ b/src/core/opamCoreConfig.ml
@@ -8,8 +8,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 module E = struct
 
   type OpamStd.Config.E.t +=
diff --git a/src/core/opamDirTrack.ml b/src/core/opamDirTrack.ml
index e556b1f34d0..c40c3b28170 100644
--- a/src/core/opamDirTrack.ml
+++ b/src/core/opamDirTrack.ml
@@ -8,7 +8,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
 open OpamStd.Op
 open OpamProcess.Job.Op
 
diff --git a/src/core/opamFilename.ml b/src/core/opamFilename.ml
index 567f019108c..011c0156813 100644
--- a/src/core/opamFilename.ml
+++ b/src/core/opamFilename.ml
@@ -400,7 +400,6 @@ let flock flag ?dontblock file = OpamSystem.flock flag ?dontblock (to_string fil
 let with_flock flag ?dontblock file f =
   let lock = OpamSystem.flock flag ?dontblock (to_string file) in
   try
-    let open OpamCompat in
     let (fd, ch) =
       match OpamSystem.get_lock_fd lock with
       | exception Not_found ->
@@ -410,14 +409,14 @@ let with_flock flag ?dontblock file f =
           else
             "/dev/null"
         in
-        let ch = Stdlib.open_out null in
+        let ch = OpamCompat.Stdlib.open_out null in
         Unix.descr_of_out_channel ch, Some ch
       | fd ->
         fd, None
     in
     let r = f fd in
     OpamSystem.funlock lock;
-    OpamStd.Option.iter Stdlib.close_out ch;
+    OpamStd.Option.iter OpamCompat.Stdlib.close_out ch;
     r
   with e ->
     OpamStd.Exn.finalise e @@ fun () ->
diff --git a/src/core/opamHash.ml b/src/core/opamHash.ml
index 3bbf3770596..bb9fb310a2a 100644
--- a/src/core/opamHash.ml
+++ b/src/core/opamHash.ml
@@ -8,8 +8,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 type kind = [ `MD5 | `SHA256 | `SHA512 ]
 
 let default_kind = `MD5
diff --git a/src/core/opamProcess.ml b/src/core/opamProcess.ml
index cde94f23868..e413068df29 100644
--- a/src/core/opamProcess.ml
+++ b/src/core/opamProcess.ml
@@ -9,8 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 let log ?level fmt =
   OpamConsole.log "PROC" ?level fmt
 
@@ -644,7 +642,7 @@ let exit_status p return =
     (verbose_print_cmd p;
      List.iter verbose_print_out stdout;
      List.iter verbose_print_out stderr;
-     flush Stdlib.stdout);
+     flush OpamCompat.Stdlib.stdout);
   let info =
     make_info ?code ?signal
       ~cmd:p.p_name ~args:p.p_args ~cwd:p.p_cwd ~metadata:p.p_metadata
diff --git a/src/core/opamStd.ml b/src/core/opamStd.ml
index c50e9264820..64d7517face 100644
--- a/src/core/opamStd.ml
+++ b/src/core/opamStd.ml
@@ -9,8 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 module type SET = sig
   include Set.S
   val map: (elt -> elt) -> t -> t
@@ -990,7 +988,7 @@ module OpamSys = struct
 
   let registered_at_exit = ref []
   let at_exit f =
-    Stdlib.at_exit f;
+    OpamCompat.Stdlib.at_exit f;
     registered_at_exit := f :: !registered_at_exit
   let exec_at_exit () =
     List.iter
@@ -1369,7 +1367,7 @@ module Exn = struct
   let finalise e f =
     let bt = Printexc.get_raw_backtrace () in
     f ();
-    Printexc.raise_with_backtrace e bt
+    OpamCompat.Printexc.raise_with_backtrace e bt
 
   let finally f k =
     match k () with
diff --git a/src/core/opamStubs.mli b/src/core/opamStubs.mli
index 1f95183133d..cbbcd67a5d2 100644
--- a/src/core/opamStubs.mli
+++ b/src/core/opamStubs.mli
@@ -89,7 +89,7 @@ val delete_glyph_checker : handle * handle -> unit
 (** Windows only. Given [(dc, font)], deletes the font object and releases the
     DC. *)
 
-val has_glyph : handle * handle -> OpamCompat.Uchar.t -> bool
+val has_glyph : handle * handle -> Uchar.t -> bool
 (** Windows only. [has_glyph (dc, font) scalar] returns [true] if [font]
     contains a glyph for [scalar].
 
diff --git a/src/core/opamSystem.ml b/src/core/opamSystem.ml
index 6e570e0e2de..02c51707583 100644
--- a/src/core/opamSystem.ml
+++ b/src/core/opamSystem.ml
@@ -9,8 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
-
 type install_warning =
   [ `Add_exe | `Install_dll | `Install_script | `Install_unknown | `Cygwin | `Cygwin_libraries ]
 type install_warning_fn = string -> install_warning -> unit
diff --git a/src/crowbar/opam-crowbar.opam b/src/crowbar/opam-crowbar.opam
index d35609dcfb3..0ebf667c5cf 100644
--- a/src/crowbar/opam-crowbar.opam
+++ b/src/crowbar/opam-crowbar.opam
@@ -8,7 +8,7 @@ homepage: "https://opam.ocaml.org"
 bug-reports: "https://github.com/ocaml/opam/issues"
 dev-repo: "git+https://github.com/ocaml/opam.git"
 depends: [
-  "ocaml" {>= "4.02.3"}
+  "ocaml" {>= "4.05.0"}
   "crowbar"
   "afl"
 ]
diff --git a/src/format/opamFile.ml b/src/format/opamFile.ml
index 7324a9e67a6..f6c7bd33757 100644
--- a/src/format/opamFile.ml
+++ b/src/format/opamFile.ml
@@ -1054,8 +1054,6 @@ module type BestEffortArg = sig
   include SyntaxFileArg
 
   (* Version of file format, as understood by [opam-file-format] *)
-  (* This attribute can be deleted when 4.02 is ditched *)
-  [@@@ocaml.warning "-32"]
   val file_format_version: OpamVersion.t [@@ocaml.warning "-32"]
 
   (* Construct the syntax pp, under some conditions. If [condition] is given,
diff --git a/src/format/opamPackage.ml b/src/format/opamPackage.ml
index 86105a1ddbb..582b25e99ca 100644
--- a/src/format/opamPackage.ml
+++ b/src/format/opamPackage.ml
@@ -9,7 +9,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
 open OpamStd.Op
 
 let log fmt = OpamConsole.log "PACKAGE" fmt
diff --git a/src/format/opamPp.ml b/src/format/opamPp.ml
index 6188ddcce26..fd5d3b68343 100644
--- a/src/format/opamPp.ml
+++ b/src/format/opamPp.ml
@@ -247,7 +247,7 @@ let fallback pp1 pp2 =
       OpamStd.Exn.fatal e;
       let bt = Printexc.get_raw_backtrace () in
       try pp2.parse ~pos x with _ ->
-        Printexc.raise_with_backtrace e bt
+        OpamCompat.Printexc.raise_with_backtrace e bt
   in
   { pp1 with parse }
 
diff --git a/src/repository/opamLocal.ml b/src/repository/opamLocal.ml
index d1a8c89cfaa..ba5bcd40bb3 100644
--- a/src/repository/opamLocal.ml
+++ b/src/repository/opamLocal.ml
@@ -10,7 +10,6 @@
 (**************************************************************************)
 
 open OpamTypes
-open OpamTypesBase
 open OpamProcess.Job.Op
 
 let log fmt = OpamConsole.log "RSYNC" fmt
diff --git a/src/repository/opamVCS.mli b/src/repository/opamVCS.mli
index af051e32c9f..82b8d98a5c0 100644
--- a/src/repository/opamVCS.mli
+++ b/src/repository/opamVCS.mli
@@ -75,4 +75,4 @@ module type VCS = sig
 end
 
 (** Create a backend from a [VCS] implementation. *)
-module Make(VCS : VCS) : OpamRepositoryBackend.S [@@ocaml.warning "-67"] (* TODO: Remove this once we get past OCaml 4.02 *)
+module Make(VCS : VCS) : OpamRepositoryBackend.S
diff --git a/src/state/dune b/src/state/dune
index 33dc0ef2a4d..ae310108174 100644
--- a/src/state/dune
+++ b/src/state/dune
@@ -4,12 +4,11 @@
   (libraries   opam-repository re)
   (synopsis    "OCaml Package Manager instance management library")
   (modules_without_implementation OpamStateTypes)
-  (modules     :standard \ flags)
+  (modules     :standard)
   (flags       (:standard
                (:include ../ocaml-flags-standard.sexp)
                (:include ../ocaml-flags-configure.sexp)
-               (:include ../ocaml-context-flags.sexp)
-               (:include flags.sexp)))
+               (:include ../ocaml-context-flags.sexp)))
   (wrapped     false))
 
 (rule
@@ -27,7 +26,6 @@
   (action (with-stdout-to %{targets}
     (progn
       (echo "(* THIS FILE IS GENERATED. See dune file *)\n\n")
-      (echo "open OpamCompat\n")
       (echo "let licenses = OpamStd.String.Set.of_list @@ List.map String.lowercase_ascii @@ [\n")
       (cat spdx-license-list)
       (echo "]\n")
@@ -46,11 +44,3 @@
     (system
       "curl https://raw.githubusercontent.com/spdx/license-list-data/master/json/exceptions.json |
        jq -rc '.exceptions | map(\"  \\\"\" + .licenseExceptionId + \"\\\";\") | join(\"\\n\")'"))))
-
-(rule
-  (with-stdout-to flags.ml
-    (echo "print_string (if String.sub Sys.ocaml_version 0 5 = \"4.02.\" then \"(-w -50)\" else \"()\")")))
-
-(rule
-  (with-stdout-to flags.sexp
-    (run ocaml %{dep:flags.ml})))
diff --git a/src/state/opamSpdxList.ml b/src/state/opamSpdxList.ml
index a3729aa78fc..478ee0fd2d8 100644
--- a/src/state/opamSpdxList.ml
+++ b/src/state/opamSpdxList.ml
@@ -1,6 +1,5 @@
 (* THIS FILE IS GENERATED. See dune file *)
 
-open OpamCompat
 let licenses = OpamStd.String.Set.of_list @@ List.map String.lowercase_ascii @@ [
   "0BSD";
   "AAL";
diff --git a/src/state/opamSysPoll.ml b/src/state/opamSysPoll.ml
index 17c45501afb..f66bb661dcc 100644
--- a/src/state/opamSysPoll.ml
+++ b/src/state/opamSysPoll.ml
@@ -8,7 +8,6 @@
 (*                                                                        *)
 (**************************************************************************)
 
-open OpamCompat
 open OpamStd.Option.Op
 
 let command_output c =
diff --git a/src_ext/dune-extlib-src b/src_ext/dune-extlib-src
index 6c5f4dd01ab..9aea1fb94e6 100644
--- a/src_ext/dune-extlib-src
+++ b/src_ext/dune-extlib-src
@@ -1,8 +1,7 @@
 (library
   (name extlib)
   (public_name extlib)
-  ; -no-strict-sequence not added until 4.03!
-  (flags (:standard \ -strict-sequence) -w -3)
+  (flags :standard -no-strict-sequence -w -3)
   (modules (:standard \ configure install base64 unzip uChar uTF8))
   (preprocess (action (run %{bin:cppo} %{read-lines:compat-level} %{input-file})))
   (wrapped false))
diff --git a/src_ext/patches/base64/bring-back-ocaml-402-support.patch b/src_ext/patches/base64/bring-back-ocaml-402-support.patch
deleted file mode 100644
index 2b47a912c69..00000000000
--- a/src_ext/patches/base64/bring-back-ocaml-402-support.patch
+++ /dev/null
@@ -1,65 +0,0 @@
-diff -Naur a/src/base64.ml b/src/base64.ml
---- a/src/base64.ml	2021-02-08 10:46:48.000000000 +0000
-+++ b/src/base64.ml	2021-07-22 17:33:45.000000000 +0000
-@@ -19,6 +19,10 @@
-  *
-  *)
- 
-+type ('a, 'b) result = ('a, 'b) Result.result =
-+  | Ok of 'a
-+  | Error of 'b
-+
- type alphabet = { emap : int array; dmap : int array }
- 
- type sub = string * int * int
-diff -Naur a/src/base64.mli b/src/base64.mli
---- a/src/base64.mli	2021-02-08 10:46:48.000000000 +0000
-+++ b/src/base64.mli	2021-07-22 17:28:56.000000000 +0000
-@@ -69,7 +69,7 @@
-   ?off:int ->
-   ?len:int ->
-   string ->
--  (sub, [> `Msg of string ]) result
-+  (sub, [> `Msg of string ]) Result.t
- (** Same as {!decode_exn} but it returns a result type instead to raise an
-     exception. Then, it returns a {!sub} string. Decoded input [(str, off, len)]
-     will starting to [off] and will have [len] bytes - by this way, we ensure to
-@@ -81,7 +81,7 @@
-   ?off:int ->
-   ?len:int ->
-   string ->
--  (string, [> `Msg of string ]) result
-+  (string, [> `Msg of string ]) Result.t
- (** Same as {!decode_exn}, but returns an explicit error message {!result} if it
-     fails. *)
- 
-@@ -91,7 +91,7 @@
-   ?off:int ->
-   ?len:int ->
-   string ->
--  (string, [> `Msg of string ]) result
-+  (string, [> `Msg of string ]) Result.t
- (** [encode s] encodes the string [s] into base64. If [pad] is false, no
-     trailing padding is added. [pad] defaults to [true], and [alphabet] to
-     {!default_alphabet}.
-@@ -109,7 +109,7 @@
-   ?off:int ->
-   ?len:int ->
-   string ->
--  (sub, [> `Msg of string ]) result
-+  (sub, [> `Msg of string ]) Result.t
- (** Same as {!encode} but return a {!sub}-string instead a plain result. By this
-     way, we ensure to allocate only one time result. *)
- 
-diff -Naur a/src/dune b/src/dune
---- a/src/dune	2021-02-08 10:46:48.000000000 +0000
-+++ b/src/dune	2021-07-22 17:28:18.000000000 +0000
-@@ -2,7 +2,7 @@
-  (name base64)
-  (modules unsafe base64)
-  (public_name base64)
-- (libraries bytes))
-+ (libraries bytes result))
- 
- (rule
-  (copy %{read:../config/which-unsafe-file} unsafe.ml))
diff --git a/src_ext/patches/cmdliner.common/0001-Restore-4.02.3-support.patch b/src_ext/patches/cmdliner.common/0001-Restore-4.02.3-support.patch
deleted file mode 100644
index 87768375ae4..00000000000
--- a/src_ext/patches/cmdliner.common/0001-Restore-4.02.3-support.patch
+++ /dev/null
@@ -1,225 +0,0 @@
-diff -Naur a/_tags b/_tags
---- a/_tags	2019-06-14 09:55:03.000000000 +0000
-+++ b/_tags	2019-11-21 14:29:33.000000000 +0000
-@@ -1,3 +1,3 @@
--true : bin_annot, safe_string
-+true : bin_annot, safe_string, package(bytes), package(result)
- <src> : include
- <test> : include
-\ No newline at end of file
-diff -Naur a/pkg/META b/pkg/META
---- a/pkg/META	2019-06-14 09:55:03.000000000 +0000
-+++ b/pkg/META	2019-11-21 14:29:33.000000000 +0000
-@@ -1,6 +1,6 @@
- version = "v1.0.4"
- description = "Declarative definition of command line interfaces"
--requires = ""
-+requires = "bytes result"
- archive(byte) = "cmdliner.cma"
- archive(native) = "cmdliner.cmxa"
- plugin(byte) = "cmdliner.cma"
-diff -Naur a/src/cmdliner.ml b/src/cmdliner.ml
---- a/src/cmdliner.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -4,11 +4,12 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+let stdlib_compare = compare
-+open Result
-+
- module Manpage = Cmdliner_manpage
- module Arg = Cmdliner_arg
- module Term = struct
--  type ('a, 'b) stdlib_result = ('a, 'b) result
--
-   include Cmdliner_term
- 
-   (* Deprecated *)
-@@ -112,7 +113,7 @@
-     ('a, [ term_escape
-          | `Exn of exn * Printexc.raw_backtrace
-          | `Parse of string
--         | `Std_help of Manpage.format | `Std_version ]) stdlib_result
-+         | `Std_help of Manpage.format | `Std_version ]) Result.result
- 
-   let run ~catch ei cl f = try (f ei cl :> 'a eval_result) with
-   | exn when catch ->
-@@ -242,7 +243,7 @@
-           Error (Cmdliner_base.err_unknown ~kind:"command" maybe ~hints)
-       | `Ambiguous ->
-           let ambs = Cmdliner_trie.ambiguities index maybe in
--          let ambs = List.sort compare ambs in
-+          let ambs = List.sort stdlib_compare ambs in
-           Error (Cmdliner_base.err_ambiguous ~kind:"command" maybe ~ambs)
- 
-   let eval_choice
-diff -Naur a/src/cmdliner.mli b/src/cmdliner.mli
---- a/src/cmdliner.mli	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner.mli	2019-11-21 14:29:33.000000000 +0000
-@@ -26,6 +26,8 @@
- 
- (** {1:top Interface} *)
- 
-+open Result
-+
- (** Man page specification.
- 
-     Man page generation is automatically handled by [Cmdliner],
-@@ -1373,6 +1375,7 @@
- 
- (* Command line interface *)
- 
-+open Result
- open Cmdliner
- 
- let lines =
-diff -Naur a/src/cmdliner_arg.ml b/src/cmdliner_arg.ml
---- a/src/cmdliner_arg.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_arg.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -6,6 +6,8 @@
- 
- let rev_compare n0 n1 = compare n1 n0
- 
-+open Result
-+
- (* Invalid_argument strings **)
- 
- let err_not_opt = "Option argument without name"
-diff -Naur a/src/cmdliner_arg.mli b/src/cmdliner_arg.mli
---- a/src/cmdliner_arg.mli	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_arg.mli	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,8 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+open Result
-+
- (** Command line arguments as terms. *)
- 
- type 'a parser = string -> [ `Ok of 'a | `Error of string ]
-diff -Naur a/src/cmdliner_base.ml b/src/cmdliner_base.ml
---- a/src/cmdliner_base.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_base.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -280,7 +280,7 @@
-   in
-   parse, print
- 
--let env_bool_parse s = match String.lowercase_ascii s with
-+let env_bool_parse s = match String.lowercase s with
- | "" | "false" | "no" | "n" | "0" -> `Ok false
- | "true" | "yes" | "y" | "1" -> `Ok true
- | s -> `Error (err_invalid_val s (alts_str ["true"; "yes"; "false"; "no" ]))
-diff -Naur a/src/cmdliner_cline.ml b/src/cmdliner_cline.ml
---- a/src/cmdliner_cline.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_cline.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,9 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+let stdlib_compare = compare
-+open Result
-+
- (* A command line stores pre-parsed information about the command
-    line's arguments in a more structured way. Given the
-    Cmdliner_info.arg values mentioned in a term and Sys.argv
-@@ -119,7 +122,7 @@
-           loop (err :: errs) (k + 1) cl pargs args
-       | `Ambiguous ->
-           let ambs = Cmdliner_trie.ambiguities optidx name in
--          let ambs = List.sort compare ambs in
-+          let ambs = List.sort stdlib_compare ambs in
-           let err = Cmdliner_base.err_ambiguous "option" name ambs in
-           loop (err :: errs) (k + 1) cl pargs args
-   in
-diff -Naur a/src/cmdliner_cline.mli b/src/cmdliner_cline.mli
---- a/src/cmdliner_cline.mli	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_cline.mli	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,8 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+open Result
-+
- (** Command lines. *)
- 
- type t
-diff -Naur a/src/cmdliner_docgen.ml b/src/cmdliner_docgen.ml
---- a/src/cmdliner_docgen.ml	2019-11-21 14:29:33.000000000 +0000
-+++ b/src/cmdliner_docgen.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -170,7 +170,7 @@
-     | true, true -> (* optional by name *)
-         let key names =
-           let k = List.hd (List.sort rev_compare names) in
--          let k = String.lowercase_ascii k in
-+          let k = String.lowercase k in
-           if k.[1] = '-' then String.sub k 1 (String.length k - 1) else k
-         in
-         compare
-@@ -178,8 +178,8 @@
-           (key @@ Cmdliner_info.arg_opt_names a1)
-     | false, false -> (* positional by variable *)
-         compare
--          (String.lowercase_ascii @@ Cmdliner_info.arg_docv a0)
--          (String.lowercase_ascii @@ Cmdliner_info.arg_docv a1)
-+          (String.lowercase @@ Cmdliner_info.arg_docv a0)
-+          (String.lowercase @@ Cmdliner_info.arg_docv a1)
-     | true, false -> -1 (* positional first *)
-     | false, true -> 1  (* optional after *)
-   in
-@@ -310,8 +310,8 @@
- 
- let title ei =
-   let main = Cmdliner_info.eval_main ei in
--  let exec = String.capitalize_ascii (Cmdliner_info.term_name main) in
--  let name = String.uppercase_ascii (invocation ~sep:'-' ei) in
-+  let exec = String.capitalize (Cmdliner_info.term_name main) in
-+  let name = String.uppercase (invocation ~sep:'-' ei) in
-   let center_header = esc @@ strf "%s Manual" exec in
-   let left_footer =
-     let version = match Cmdliner_info.term_version main with
-diff -Naur a/src/cmdliner_manpage.ml b/src/cmdliner_manpage.ml
---- a/src/cmdliner_manpage.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_manpage.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,8 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+open Result
-+
- (* Manpages *)
- 
- type block =
-diff -Naur a/src/cmdliner_term.ml b/src/cmdliner_term.ml
---- a/src/cmdliner_term.ml	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_term.ml	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,8 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+open Result
-+
- type term_escape =
-   [ `Error of bool * string
-   | `Help of Cmdliner_manpage.format * string option ]
-diff -Naur a/src/cmdliner_term.mli b/src/cmdliner_term.mli
---- a/src/cmdliner_term.mli	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/cmdliner_term.mli	2019-11-21 14:29:33.000000000 +0000
-@@ -4,6 +4,8 @@
-    cmdliner v1.0.4
-   ---------------------------------------------------------------------------*)
- 
-+open Result
-+
- (** Terms *)
- 
- type term_escape =
-diff -Naur a/src/dune b/src/dune
---- a/src/dune	2019-06-14 09:55:03.000000000 +0000
-+++ b/src/dune	2019-11-21 14:29:33.000000000 +0000
-@@ -1,4 +1,5 @@
- (library
-  (public_name cmdliner)
-+ (libraries result)
-  (flags :standard -w -3-6-27-32-35)
-  (wrapped false))
diff --git a/src_ext/patches/dose3/bring-back-ocaml-402-support.patch b/src_ext/patches/dose3/bring-back-ocaml-402-support.patch
deleted file mode 100644
index 77a3f728a50..00000000000
--- a/src_ext/patches/dose3/bring-back-ocaml-402-support.patch
+++ /dev/null
@@ -1,12 +0,0 @@
-diff -Naur a/src/common/util.ml b/src/common/util.ml
---- a/src/common/util.ml	2021-07-22 07:14:19.000000000 +0000
-+++ b/src/common/util.ml	2021-07-22 17:34:13.000000000 +0000
-@@ -90,7 +90,7 @@
-   let clean label =
-     try
-       let s = Filename.chop_extension (Filename.basename label) in
--      String.capitalize_ascii s
-+      String.capitalize s
-     with Invalid_argument _ -> label
- 
-   let create ?(enabled = false) label =
diff --git a/src_ext/patches/ocamlgraph/restore-4.02-support.patch b/src_ext/patches/ocamlgraph/restore-4.02-support.patch
deleted file mode 100644
index 90efc6c8c4d..00000000000
--- a/src_ext/patches/ocamlgraph/restore-4.02-support.patch
+++ /dev/null
@@ -1,23 +0,0 @@
-diff -Naur a/src/dot_lexer.mll b/src/dot_lexer.mll
---- a/src/dot_lexer.mll	2020-10-02 13:47:57.000000000 +0000
-+++ b/src/dot_lexer.mll	2021-03-26 12:32:56.000000000 +0000
-@@ -35,7 +35,7 @@
- 	"node", NODE;
- 	"edge", EDGE;
-       ];
--    fun s -> let s = String.lowercase_ascii s in Hashtbl.find h s
-+    fun s -> let s = String.lowercase s in Hashtbl.find h s
- 
- }
- 
-diff -Naur a/src/dune b/src/dune
---- a/src/dune	2020-10-02 13:47:57.000000000 +0000
-+++ b/src/dune	2021-03-26 12:33:07.000000000 +0000
-@@ -3,6 +3,7 @@
- (library
-  (name graph)
-  (public_name ocamlgraph)
-+ (flags :standard -w -3)
-  (libraries stdlib-shims)
-  (modules_without_implementation dot_ast sig sig_pack))
-