I saw that https://github.com/sidharthkuruvila/ocaml-ide was pretty recent and assumed that it worked. I haven't tried it though.

Amusingly someone made a plugin for pg's Arc 3.1 code back in '08: https://i.imgur.com/Xe7mvIB.png

If IDEA doesn't support a particular language, it's pretty easy to fork an existing plugin and throw together some lexer rules.