You already know that there's an O2 sensor problem. Fix that first. See what happens.
How long has it been since the vehicle got a proper "Tune-Up"? Distributor cap problems seem to be common, as are distributor gears that have worn-out, and distributor housings that have sloppy bushings. All of this affects timing and spark delivery.
What is the fuel pressure? Still have the crappy "Spider" injectors, or has it gotten the upgrade to electronic injectors?
As always, connect a scan tool and look at the data stream. Can you narrow down the misfire to a specific cylinder? Fuel trims look OK? Coolant temp sensor, MAP sensor, TPS giving good readings? Anything else in the data stream look "odd"?