OCaml.

06Installing OCaml with opam

Install opam, OCaml's package manager, and let it build the compiler: opam init creates a default switch, an isolated environment holding one compiler and its packages. opam has supported Windows natively since 2.2.0; the current opam release is 2.6.1, from 7 October 2026.

1. On Linux, macOS or *BSD, make sure gcc, build-essential, curl, unzip and bubblewrap are installed, then run the install script and initialise opam:

   bash -c "sh <(curl -fsSL https://opam.ocaml.org/install.sh)"
   opam init
   eval $(opam env)

2. On Windows, install Git and opam with winget, then run opam init in a new terminal:

   winget install Git.Git OCaml.opam

3. Add the OCaml Platform tools to the switch: Dune for builds, the language server for editors, odoc, ocamlformat and the utop REPL:

   opam install dune ocaml-lsp-server odoc ocamlformat utop

If you said no to the shell hook during opam init, run eval $(opam env) in each new shell, or prefix commands with opam exec --. Distribution packages of opam can lag behind and may not know the newest compiler.

Install OCaml / opam releases

07Which OCaml version to run, and moving to a new one

New projects should use the newest 5.x release. The 4.14 line still gets patch releases for code that can't move to OCaml 5's multicore runtime yet; the chart shows every compiler release since 2024 by series.

1. Refresh the package list and create a switch for the new compiler. A switch named after a directory is picked automatically when you run opam inside it:

   opam update
   opam switch create 5.5.2
   eval $(opam env)

2. Reinstall your project's dependencies in the new switch, for example with opam install . --deps-only from a directory with an opam file, then rebuild and run your tests.
3. Remove the old switch once nothing needs it: opam switch remove <name>.

Creating a switch compiles OCaml from source, which takes a few minutes; the ocaml-system package reuses a compiler installed by your OS instead.

opam usage / OCaml releases / opam switch manual / opam install manual

08Pinning a package with opam

opam pin add <package> <target> holds a package at a version, or points it at a local path or a git branch or commit, and opam upgrade then leaves it alone or rebuilds it from that source.

1. Pin to a version, a checkout, a git ref or the package's upstream repository:

   opam pin add camlpdf 1.7
   opam pin add camlpdf ~/src/camlpdf
   opam pin add opam-lib https://github.com/ocaml/opam.git#1.2
   opam pin add opam-lib --dev-repo

2. To patch a dependency, fetch its source pinned, edit it, and rebuild from the checkout:

   opam source <package> --dev-repo --pin
   cd <package>
   opam upgrade .

3. List pins with opam pin list and drop one with opam pin remove <package>.

opam usage: opam pin / opam pin manual