diff --git a/pkgs/development/ocaml-modules/0install-solver/default.nix b/pkgs/development/ocaml-modules/0install-solver/default.nix
new file mode 100644
index 000000000000..2801fbf47b89
--- /dev/null
+++ b/pkgs/development/ocaml-modules/0install-solver/default.nix
@@ -0,0 +1,30 @@
+{
+  lib,
+  buildDunePackage,
+  fetchFromGitHub,
+  ounit2,
+}:
+
+buildDunePackage (finalAttrs: {
+  pname = "0install-solver";
+  version = "2.18";
+
+  src = fetchFromGitHub {
+    owner = "0install";
+    repo = "0install";
+    tag = "v${finalAttrs.version}";
+    hash = "sha256-CxADWMYZBPobs65jeyMQjqu3zmm2PgtNgI/jUsYUp8I=";
+  };
+
+  doCheck = true;
+  checkInputs = [
+    ounit2
+  ];
+
+  meta = {
+    description = "Package dependency solver";
+    license = lib.licenses.lgpl21;
+    maintainers = [ lib.maintainers.stepbrobd ];
+    homepage = "https://0install.net";
+  };
+})
diff --git a/pkgs/development/ocaml-modules/opam-0install-cudf/default.nix b/pkgs/development/ocaml-modules/opam-0install-cudf/default.nix
new file mode 100644
index 000000000000..3fc260f43ead
--- /dev/null
+++ b/pkgs/development/ocaml-modules/opam-0install-cudf/default.nix
@@ -0,0 +1,50 @@
+{
+  lib,
+  buildDunePackage,
+  fetchFromGitHub,
+  _0install-solver,
+  alcotest,
+  cudf,
+}:
+
+buildDunePackage (finalAttrs: {
+  pname = "opam-0install-cudf";
+  version = "0.5.0";
+
+  src = fetchFromGitHub {
+    owner = "ocaml-opam";
+    repo = "opam-0install-cudf";
+    rev = "v${finalAttrs.version}";
+    hash = "sha256-TETfvR1Di4c8CylsKnMal/GfQcqMSr36o7511u1bYYs=";
+  };
+
+  propagatedBuildInputs = [
+    cudf
+    _0install-solver
+  ];
+
+  doCheck = true;
+  checkInputs = [
+    alcotest
+  ];
+
+  meta = {
+    homepage = "https://github.com/ocaml-opam/opam-0install-cudf";
+    description = "Opam solver using 0install backend using the CUDF interface";
+    longDescription = ''
+      Opam's default solver is designed to maintain a set of packages
+      over time, minimising disruption when installing new programs and
+      finding a compromise solution across all packages.
+
+      In many situations (e.g. CI, local roots or duniverse builds) this
+      is not necessary, and we can get a solution much faster by using
+      a different algorithm.
+
+      This package provides a generic solver library which uses 0install's
+      solver library. The library uses the CUDF library in order to interface
+      with opam as it is the format common used to talk to all the supported solvers.
+    '';
+    license = lib.licenses.isc;
+    maintainers = [ lib.maintainers.stepbrobd ];
+  };
+})
diff --git a/pkgs/development/ocaml-modules/opam-client/default.nix b/pkgs/development/ocaml-modules/opam-client/default.nix
new file mode 100644
index 000000000000..9840c87c2894
--- /dev/null
+++ b/pkgs/development/ocaml-modules/opam-client/default.nix
@@ -0,0 +1,33 @@
+{
+  lib,
+  buildDunePackage,
+  base64,
+  cmdliner,
+  opam,
+  opam-repository,
+  opam-solver,
+  opam-state,
+  re,
+}:
+
+buildDunePackage {
+  pname = "opam-client";
+
+  inherit (opam) src version;
+
+  configureFlags = [ "--disable-checks" ];
+
+  propagatedBuildInputs = [
+    base64
+    cmdliner
+    opam-repository
+    opam-solver
+    opam-state
+    re
+  ];
+
+  meta = opam.meta // {
+    description = "Actions on the opam root, switches, installations, and front-end";
+    maintainers = with lib.maintainers; [ stepbrobd ];
+  };
+}
diff --git a/pkgs/development/ocaml-modules/opam-solver/default.nix b/pkgs/development/ocaml-modules/opam-solver/default.nix
new file mode 100644
index 000000000000..8dd328cda640
--- /dev/null
+++ b/pkgs/development/ocaml-modules/opam-solver/default.nix
@@ -0,0 +1,35 @@
+{
+  lib,
+  buildDunePackage,
+  cudf,
+  dose3,
+  mccs,
+  opam,
+  opam-0install-cudf,
+  opam-format,
+  re,
+  z3,
+}:
+
+buildDunePackage {
+  pname = "opam-solver";
+
+  inherit (opam) src version;
+
+  configureFlags = [ "--disable-checks" ];
+
+  propagatedBuildInputs = [
+    cudf
+    dose3
+    mccs
+    opam-0install-cudf
+    opam-format
+    re
+    z3
+  ];
+
+  meta = opam.meta // {
+    description = "This library is based on the Cudf and Dose libraries, and handles calls to the external solver from opam";
+    maintainers = with lib.maintainers; [ stepbrobd ];
+  };
+}
diff --git a/pkgs/development/ocaml-modules/oui/default.nix b/pkgs/development/ocaml-modules/oui/default.nix
new file mode 100644
index 000000000000..98c9d2e12c73
--- /dev/null
+++ b/pkgs/development/ocaml-modules/oui/default.nix
@@ -0,0 +1,60 @@
+{
+  lib,
+  buildDunePackage,
+  fetchFromGitHub,
+  cmdliner,
+  fmt,
+  markup,
+  ocaml-crunch,
+  opam-client,
+  opam-format,
+  ppx_deriving_yojson,
+  ppx_expect,
+  yojson,
+}:
+
+buildDunePackage (finalAttrs: {
+  pname = "oui";
+  version = "0-unstable-2025-10-08";
+
+  minimalOCamlVersion = "4.10";
+
+  src = fetchFromGitHub {
+    owner = "OCamlPro";
+    repo = "ocaml-universal-installer";
+    rev = "2fe2e33c3f8e1744fdd4dab04458043451bf9f62";
+    hash = "sha256-ALQIQ3Ab1Gs2xST9OwsO5IxixzgKlUg7uHZvfHMbv7Q=";
+  };
+
+  nativeBuildInputs = [
+    ocaml-crunch
+  ];
+
+  propagatedBuildInputs = [
+    cmdliner
+    fmt
+    markup
+    opam-client
+    opam-format
+    ppx_deriving_yojson
+    yojson
+  ];
+
+  # upstream tests are matching full library paths
+  doCheck = false;
+  checkInputs = [
+    ppx_expect
+  ];
+
+  meta = {
+    homepage = "https://github.com/OCamlPro/ocaml-universal-installer";
+    description = "A tool that builds standalone installers for your OCaml applications";
+    longDescription = ''
+      oui is a command-line tool that generates standalone installers for
+      Linux, Windows and macOS from your OCaml and non OCaml binaries.
+    '';
+    license = lib.licenses.lgpl21;
+    maintainers = [ lib.maintainers.stepbrobd ];
+    mainProgram = "opam-oui";
+  };
+})
diff --git a/pkgs/top-level/ocaml-packages.nix b/pkgs/top-level/ocaml-packages.nix
index ccbe15b7ee9b..6463b9a32430 100644
--- a/pkgs/top-level/ocaml-packages.nix
+++ b/pkgs/top-level/ocaml-packages.nix
@@ -1580,16 +1580,22 @@ let
 
         omd = callPackage ../development/ocaml-modules/omd { };
 
+        opam-0install-cudf = callPackage ../development/ocaml-modules/opam-0install-cudf { };
+
         opam-core = callPackage ../development/ocaml-modules/opam-core {
           inherit (pkgs) opam;
         };
 
+        opam-client = callPackage ../development/ocaml-modules/opam-client { };
+
         opam-file-format = callPackage ../development/ocaml-modules/opam-file-format { };
 
         opam-format = callPackage ../development/ocaml-modules/opam-format { };
 
         opam-repository = callPackage ../development/ocaml-modules/opam-repository { };
 
+        opam-solver = callPackage ../development/ocaml-modules/opam-solver { };
+
         opam-state = callPackage ../development/ocaml-modules/opam-state { };
 
         opium = callPackage ../development/ocaml-modules/opium { };
@@ -1614,6 +1620,8 @@ let
 
         otr = callPackage ../development/ocaml-modules/otr { };
 
+        oui = callPackage ../development/ocaml-modules/oui { };
+
         ounit = callPackage ../development/ocaml-modules/ounit { };
 
         ounit2 = callPackage ../development/ocaml-modules/ounit2 { };
@@ -2219,6 +2227,10 @@ let
 
         zmq-lwt = callPackage ../development/ocaml-modules/zmq/lwt.nix { };
 
+        ### _ ###
+
+        _0install-solver = callPackage ../development/ocaml-modules/0install-solver { };
+
         ### Exceptional packages kept out of order ###
 
         # Libs