{
  # Use this repo as the `nixpkgs` URL
  inputs.nixpkgs.url = "github:nix-ocaml/nix-overlays";


  outputs = { self, nixpkgs }:
    let
      pkgs = nixpkg.legacyPackages.${"YOUR_SYSTEM_STRING"};
    in
    {
      ...
    };
}

{
  inputs.nixpkgs.url = "github:nixOS/nixpkgs";

  inputs.ocaml-overlay.url = "github:nix-ocaml/nix-overlays";
  inputs.ocaml-overlay.inputs.nixpkgs.follows = "nixpkgs";


  outputs = { self, nixpkgs, ocaml-overlay }:
    let
      system = "YOUR_SYSTEM_STRING";
      pkgs = import nixpkgs {
        inherit system;
        overlays = [
          ocaml-overlay.overlays.default
        ];
      };
    in
    {
      ...
    };
}

let
  nixpkgs-sources =
    builtins.fetchTarball
      https://github.com/nix-ocaml/nix-overlays/archive/master.tar.gz;
  pkgs = import nixpkgs-sources { };
in

pkgs

let
  nixpkgs-sources =
    builtins.fetchTarball
      https://github.com/nix-ocaml/nix-overlays/archive/master.tar.gz;

  custom-nixpkgs = /path/to/custom/nixpkgs;

  pkgs = import custom-nixpkgs {
    overlays = [
      (import "${nixpkgs-sources}/overlay" custom-nixpkgs)
    ];
  };
in

pkgs