My 2001 chevy suburban feels heavy like it wont shift

Check transmission fluid level,could be very low transmission slipping not going into over drive.if been long time change transmission fluid and filter.if transmission fluid filter already been changed other problems cause transmission not to shift could have worn broken need adjusting.governor valve sticking, governor drive gear worn, governor weights or springs missing or damaged. you could have sticking first and second shift valves.your first and second servos pistons seals stuck or damaged. do fluid change and add some high mileage transmission treatment go to any auto parts stores find transmission additive that will loosen up transmission sticking shift valves in the valve body. if your check engine light is on have vechicle code scan for faulty tps in which will cause transmission not to shift over drive.

Try disconnecting the Negative battery cable for 30 minutes, then reconnect. It may help to reboot the system to recognize the new part.

The O2 sensor should not have that extreme of an effect on the transmission. Even a limp home mode would allow some of the gears to work

When you went under the vehicle to work on it, did you set the Parking Brake? Chances are that if you did, it may not have released and is dragging.