Empire of Angels IV

I'll try to explain this without any spoilers. This achievement seems bugged, but it's not. It's just not worded correctly. You can upgrade Kefir to Oracle pretty early in the game and you won't get the achievement. That's fine. There's another "Oracle" related upgrade she is given much later in the game, and that's what unlocks the achievement.

there is 2 different upgrade options then, but it's kind of complicated and I don't know exactly how it works, but you're basically guaranteed to get one of them and the achievement with it, so just keep playing and you'll get it eventually.