oj is equivalent to oji, and it's the macrolanguage. We usually avoid adding them, although sometimes we do it, when there is a particular reason. If there is such a reason here, I'm happy to hear it. It should probably be in another task. It probably won't be in Names.php, as the title of T277505 asks, because Names.php only lists languages that passed a threshold of core MediaWiki localization, but it can be considered for Wikidata values, for example.
oj is needed in Wikispecies: https://species.wikimedia.org/wiki/Template_talk:VN#cr_and_oj - for which , I have been told, Names.php is the appropriate solution.
It should probably be in another task