For OCaml, all the syntax highlighting on GitHub is done via this project:
It doesn’t appear as though this is a very active project however, so I am wondering if it would be worth it for the OCaml organization to take over and maintain this repo and eventually point github/linguist to the maintained one.
Their grammar format is slightly different (JSON instead of plist), but according to my research it is supported by Linguist too.
Now, it’s a matter of following the steps in the Linguist documentation to point to the new repo and run the checks that they have, fix any issues and submit a PR to the Linguist, but I haven’t gotten to it. Any volunteers?
Also, the folks @ocamllabs should be consulted, because once this is done, a change in their highlighting grammar will be automatically applied (at regular intervals—a couple of times per year, I reckon) to the whole GitHub source code, and that puts a bit of pressure on them.
Github uses tree-sitter grammars for some languages, which would be possible for OCaml too with better results than the textmate grammars. But they don’t have a public process to add new tree-sitter grammars.