Why is my Subaru misfiring?

2001 Subaru Impreza Outback Sport.

Bought the car 7 months ago. 244000 miles. Changed crank sensor, plugs, wires, and coil packs. And its rough idling and no power now. Oily spark plugs. S1 B1 code. But can't get the sensor out. And I've been told to check spark. All 4 spark normally. And don't really wanna put in shop for the o2 sensor. But is there anything else it could be?