There's a fatal flaw in this premise.
When she gets horny, her horns come out.
The more horny she gets, the bigger the horns get.
The bigger the horns get, the stronger and more dangerous she becomes... because her strength gets more and more uncontrollable as it keeps increasing.

Oni kouhai will never be able to make love to her senpai without accidentally killing her. >_<

Solution: Senpai is an oni too, but only finds out after being confessed to by oni kouhai.