1996 Camry spark plug popped out and there is oil inside?

I checked the oil on the car the other day and it was low. Wile filling it up saw that one
of the spark plugs is out. Pulled it out and there is oil in there. After a few days the car
started choking on the free way and the engine light started blinking. Not too long ago
from Autozone scan they told me that there is a Air leak on the exhaust, Faulty O2
sensor or catalytic converter.  What would the reason for oil in spark plugs be? Also
how can it be fixed and how much would it cost? I have the car parked at home and
scared to drive it. Any suggestions? Thank you!