In a way the Akito-Oshiko relationship feels like a training wheel relationship for Akito, before he matures enough to enter into an Akito-Mahiru relationship. But while this would fit on the face of it, it doesn't feel like Oshiko is approaching their relationship in that teaching/maturing way. If she were, it would be a bit disingenious, if that was her intent, to give Akito a chance to learn how to relationship before entering a "proper" one. She is more outgoing and gives the impression of being more experienced romantically, but we don't actually know that.
Rather it feels like Oshiko has an honest and genuine interest in a relationship with Akito, and just happens to be the more proactive and outgoing type between her and Miharu to actually pursue a relationship despite their previous acquaintance being comparatively surface level.
So, for what it's worth, I could see her being in a relationship with Akito for a while and then stepping back for the other two to proceed in their courtship, not because she doesn't want a relationship herself with him, but rather in acknowledgement that the two, Miharu and Akito, would be making each other happy in a way she as a friend to one and partner to the other wouldn't be capable of to meet, neither in kind nor degree.