If you have a vacuum leak, then it should have a high idle.
As far as a bad O2 sensor, it would cause the motor to run rich, but not enough to put gas in the oil.
I would think that if you pulled the spark plugs they would tell you which cylinder has a sticking injector. Sounds to me like an injector is stuck open and just dumping fuel into that cylinder.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...