diff --git a/ocamltest/actions_helpers.ml b/ocamltest/actions_helpers.ml
index 4c216f8be..7ce407f39 100644
--- a/ocamltest/actions_helpers.ml
+++ b/ocamltest/actions_helpers.ml
@@ -68,7 +68,7 @@ let setup_symlinks test_source_directory build_directory files =
     let src = Filename.concat test_source_directory filename in
     let dst = Filename.concat build_directory filename in
     Sys.copy_file src dst in
-  let f = if Sys.os_type="Win32" then copy else symlink in
+  let f = if Sys.win32 then copy else symlink in
   Sys.make_directory build_directory;
   List.iter f files
 
@@ -121,7 +121,7 @@ let run_cmd
   in
   let lst = List.concat (List.map String.words cmd) in
   let quoted_lst =
-    if Sys.os_type="Win32"
+    if Sys.win32
     then List.map Filename.maybe_quote lst
     else lst in
   let cmd' = String.concat " " quoted_lst in
diff --git a/ocamltest/filecompare.ml b/ocamltest/filecompare.ml
index 2e87d0ce0..7bfdb20f6 100644
--- a/ocamltest/filecompare.ml
+++ b/ocamltest/filecompare.ml
@@ -138,11 +138,8 @@ let compare_files ?(tool = default_comparison_tool) files =
         files.reference_filename;
         files.output_filename
       ] in
-      let dev_null = match Sys.os_type with
-        | "Win32" -> "NUL"
-        | _ -> "/dev/null" in
       let settings = Run_command.settings_of_commandline
-        ~stdout_fname:dev_null ~stderr_fname:dev_null commandline in
+        ~stdout_fname:Filename.null ~stderr_fname:Filename.null commandline in
       let status = Run_command.run settings in
       result_of_exitcode commandline status
   | Internal ignore ->
diff --git a/ocamltest/ocaml_actions.ml b/ocamltest/ocaml_actions.ml
index dc363f297..a2de63bd5 100644
--- a/ocamltest/ocaml_actions.ml
+++ b/ocamltest/ocaml_actions.ml
@@ -852,16 +852,14 @@ let really_compare_programs backend comparison_tool log env =
       "flambda temporarily disables comparison of native programs" in
     (Result.pass_with_reason reason, env)
   end else
-  if backend = Ocaml_backends.Native &&
-    (Sys.os_type="Win32" || Sys.os_type="Cygwin")
+  if backend = Ocaml_backends.Native && (Sys.win32 || Sys.cygwin)
   then begin
     let reason =
       "comparison of native programs temporarily disabled under Windows" in
     (Result.pass_with_reason reason, env)
   end else begin
     let comparison_tool =
-      if backend=Ocaml_backends.Native &&
-        (Sys.os_type="Win32" || Sys.os_type="Cygwin")
+      if backend=Ocaml_backends.Native && (Sys.win32 || Sys.cygwin)
         then
           let bytes_to_ignore = 512 (* comparison_start_address program *) in
           Filecompare.(make_cmp_tool ~ignore:{bytes=bytes_to_ignore; lines=0})
diff --git a/ocamltest/ocaml_directories.ml b/ocamltest/ocaml_directories.ml
index 8ec499349..9f1c84652 100644
--- a/ocamltest/ocaml_directories.ml
+++ b/ocamltest/ocaml_directories.ml
@@ -24,7 +24,7 @@ let stdlib =
   Filename.make_path [srcdir; "stdlib"]
 
 let libunix =
-  let subdir = if Sys.os_type="Win32" then "win32unix" else "unix" in
+  let subdir = if Sys.win32 then "win32unix" else "unix" in
   Filename.make_path [srcdir; "otherlibs"; subdir]
 
 let toplevel =
diff --git a/ocamltest/ocaml_modifiers.ml b/ocamltest/ocaml_modifiers.ml
index c310cf362..ae3ad8d33 100644
--- a/ocamltest/ocaml_modifiers.ml
+++ b/ocamltest/ocaml_modifiers.ml
@@ -79,7 +79,7 @@ let testing = make_library_modifier
 let tool_ocaml_lib = make_module_modifier
   "lib" (compiler_subdir ["testsuite"; "lib"])
 
-let unixlibdir = if Sys.os_type="Win32" then "win32unix" else "unix"
+let unixlibdir = if Sys.win32 then "win32unix" else "unix"
 
 let unix = make_library_modifier
   "unix" (compiler_subdir ["otherlibs"; unixlibdir])
diff --git a/ocamltest/ocamltest_stdlib.ml b/ocamltest/ocamltest_stdlib.ml
index 90ba02a54..3ee6d104e 100644
--- a/ocamltest/ocamltest_stdlib.ml
+++ b/ocamltest/ocamltest_stdlib.ml
@@ -28,7 +28,7 @@ end
 
 module Filename = struct
   include Filename
-  let path_sep = if Sys.os_type="Win32" then ";" else ":"
+  let path_sep = if Sys.win32 then ";" else ":"
   (* This function comes from otherlibs/win32unix/unix.ml *)
   let maybe_quote f =
     if String.contains f ' ' ||
@@ -43,7 +43,7 @@ module Filename = struct
   let make_path components = List.fold_left Filename.concat "" components
 
   let mkexe =
-    if Sys.os_type="Win32"
+    if Sys.win32
     then fun name -> make_filename name "exe"
     else fun name -> name
 end
diff --git a/ocamltest/run_command.ml b/ocamltest/run_command.ml
index 1a1df6147..5819fa662 100644
--- a/ocamltest/run_command.ml
+++ b/ocamltest/run_command.ml
@@ -32,7 +32,7 @@ type settings = {
 let settings_of_commandline ?(stdout_fname="") ?(stderr_fname="") commandline =
   let words = String.words commandline in
   let quoted_words =
-    if Sys.os_type="Win32"
+    if Sys.win32
     then List.map Filename.maybe_quote words
     else words in
   {