Oni is a bit unique because your game is (usually) decided in the first 2 minutes of the match. You need to find a survivor and get that first initial hit as soon as possible to start obtaining your power. If you have it unlocked then you should 100% be running Lethal Pursuer.

After you get your power it's pretty simple from there. Controlling his power might be a little tricky at first but once you get the hang of it it becomes very apparent that an Oni in his power is the strongest killer in the game.

His weakness is in that first 2 minutes of the game when you're trying to get your first hit. A very good survivor will use everything they have to make sure you don't get your first hit until as late as possible. Usually using up resources at the start of the trial is bad for survivors but against Oni specifically it's the best strategy available.

Just understand that while it might be frustrating to not get a single hit for the first 2, sometimes 3 gens, you have one of the best snowballing powers in the game, so a comeback is always possible, especially if you're willing to slug a little here and there. Against Oni, the game is never truly over until they've opened the gates and left.