You need to be smart when you fight with Oni on your team. His passive ability works great only if he is not the last one of your characters and also if the opponent is the last one of his team. That way the opponent will lose a bar of power if he defeats Oni and he cannot tag out. Apart from that, Oni has one of the highest health and attack from the bronze characters.
Demonic Curse - Power Drain: When defeated, the opponent loses 1 bar of power.
Singe - Attack: Medium damage
Bounded - Attack: High damage + Blind
Demonic Fury - Bonus Attack in percentage for all Oni characters
Demonic Toughness - Bonus Health for all Oni characters
Netherrealm Creature - Bonus Recovery for all Oni characters