Sounds like it may be burning oil that is getting past the rings. This would damage the oxygen sensor and cause bad gas mileage. So, the rings are bad which really means you are close to needing a rebuild. You probably don't have good compression. Replacing the oxygen sensor will not help for long as it will damage the new one quickly.
Stuck injector, clogged air filter, fuel pressure too high? Plus many more items. Is there any codes in the system