Could be O2... or it could be the tps sensor, or fuel injector circuit problem. Pull up the carpet and check out your codes, then post them here for a better diagnosis.
As far as not starting, so it cranks but doesn't start? Timing could definitely be a culprit, so if u think it's off, then check it with a timing gun. I would also check everything electrical, then move to fuel. Check spark plugs, wires, cap, rotor, ignition circuit, and if all that's okay, then check your fuel pump and fuel injectors for proper operation. Could be a main relay problem...search and you'll find plenty on the notorious main relay.