This is partially a Github issue. But it seems the only way for adding a language is to submit your lexer:
Is there a lexer for Opa yet that can be submitted?
Right now we get no highlighting and does not recognize it as the Opa language in the lists of languages used or the other places places it lists the main language used for a project.
Very good point. Github uses Pygments for syntax highlighting. For a long time we have a Pygments-compatible lexer for Opa. Many months ago I tried (more than once) to contact Pygments developers but until now I received no answer whatsoever. I'll try again now and if nothing happens maybe will ask the Opa community to put some pressure on them :).
Update: I just sent a pull request to Pygments authors, https://bitbucket.org/birkenfeld/pygments-main/pull-requests. Let's hope they'll react on it soon and then we can move forward on this issue.
Update: Opa has been accepted in Pygments and is now listed as a language in GitHub, however, no coloration...