diff --git a/pkgs/development/compilers/opa/default.nix b/pkgs/development/compilers/opa/default.nix
index 5e648c9de4ad..8e70ca8dac25 100644
--- a/pkgs/development/compilers/opa/default.nix
+++ b/pkgs/development/compilers/opa/default.nix
@@ -15,7 +15,7 @@ stdenv.mkDerivation rec {
     sha256 = "1qs91rq9xrafv2mf2v415k8lv91ab3ycz0xkpjh1mng5ca3pjlf3";
   };
 
-  patches = [ ./ocaml-4.03.patch ];
+  patches = [ ./ocaml-4.03.patch ./ocaml-4.04.patch ];
 
   # Paths so the opa compiler code generation will use the same programs as were
   # used to build opa.
diff --git a/pkgs/development/compilers/opa/ocaml-4.04.patch b/pkgs/development/compilers/opa/ocaml-4.04.patch
new file mode 100644
index 000000000000..45cae411fb34
--- /dev/null
+++ b/pkgs/development/compilers/opa/ocaml-4.04.patch
@@ -0,0 +1,75 @@
+diff --git a/compiler/libbsl/bslLib.ml b/compiler/libbsl/bslLib.ml
+index b9f75bd1..171af065 100644
+--- a/compiler/libbsl/bslLib.ml
++++ b/compiler/libbsl/bslLib.ml
+@@ -726,7 +726,7 @@ struct
+       let root elt = !(elt.root)
+       let elt_name elt = elt.name
+       let elts e = e
+-      let children = List.map (fun e -> e.name, e)
++      let children m = List.map (fun e -> e.name, e) m
+       let is_root e = Path.is_root e.pwd
+ 
+       let parent e =
+diff --git a/compiler/passes/surfaceAstDependencies.ml b/compiler/passes/surfaceAstDependencies.ml
+index f4354a3f..81253d32 100644
+--- a/compiler/passes/surfaceAstDependencies.ml
++++ b/compiler/passes/surfaceAstDependencies.ml
+@@ -66,7 +66,6 @@ open SurfaceAst
+ 
+ (* shorthands *)
+ module SAH = SurfaceAstHelper
+-module C = SurfaceAstCons.ExprIdentCons
+ module D = SurfaceAstDecons
+ module S = SurfaceAst
+ 
+diff --git a/compiler/passes/surfaceAstPasses.ml b/compiler/passes/surfaceAstPasses.ml
+index 10edf5cb..00de59fa 100644
+--- a/compiler/passes/surfaceAstPasses.ml
++++ b/compiler/passes/surfaceAstPasses.ml
+@@ -25,7 +25,6 @@ open SurfaceAstPassesTypes
+ 
+ (* alias *)
+ module C = SurfaceAstCons.ExprIdentCons
+-module CS = SurfaceAstCons.StringCons
+ 
+ 
+ 
+diff --git a/compiler/qmlslicer/qmlSimpleSlicer.ml b/compiler/qmlslicer/qmlSimpleSlicer.ml
+index 2eebd96b..04ce77c8 100644
+--- a/compiler/qmlslicer/qmlSimpleSlicer.ml
++++ b/compiler/qmlslicer/qmlSimpleSlicer.ml
+@@ -17,7 +17,6 @@
+ *)
+ module Format = Base.Format
+ module List = Base.List
+-module String = Base.String
+ module Q = QmlAst
+ module Package = ObjectFiles.Package
+ 
+diff --git a/ocamllib/libbase/baseObj.mli b/ocamllib/libbase/baseObj.mli
+index da2d9736..82d72963 100644
+--- a/ocamllib/libbase/baseObj.mli
++++ b/ocamllib/libbase/baseObj.mli
+@@ -21,7 +21,7 @@ type t = Obj.t
+ external repr : 'a -> t = "%identity"
+ external obj : t -> 'a = "%identity"
+ external magic : 'a -> 'b = "%identity"
+-external is_block : t -> bool = "caml_obj_is_block"
++val [@inline always] is_block : t -> bool
+ external is_int : t -> bool = "%obj_is_int"
+ external tag : t -> int = "caml_obj_tag"
+ external set_tag : t -> int -> unit = "caml_obj_set_tag"
+diff --git a/ocamllib/libbase/baseString.ml b/ocamllib/libbase/baseString.ml
+index 640ce2fa..6931c608 100644
+--- a/ocamllib/libbase/baseString.ml
++++ b/ocamllib/libbase/baseString.ml
+@@ -20,7 +20,7 @@
+ (* depends *)
+ module Char = BaseChar
+ 
+-include Bytes
++include String
+ 
+ let compare_int (a:int) b = Pervasives.compare a b
+ 
diff --git a/pkgs/top-level/all-packages.nix b/pkgs/top-level/all-packages.nix
index 72fdc8cbbc4e..a6f8762abcb9 100644
--- a/pkgs/top-level/all-packages.nix
+++ b/pkgs/top-level/all-packages.nix
@@ -10827,7 +10827,7 @@ in
   ocsigen-i18n = callPackage ../development/tools/ocaml/ocsigen-i18n { };
 
   opa = callPackage ../development/compilers/opa {
-    ocamlPackages = ocaml-ng.ocamlPackages_4_03;
+    ocamlPackages = ocaml-ng.ocamlPackages_4_04;
   };
 
   opaline = callPackage ../development/tools/ocaml/opaline { };