Engine wont engage

Hi! I own a 2002 Chevy Tracker with 75k miles on it. I recently had the PCV Valve and Starter replaced, along with the fuel lines. I went to start it this morning and it would turn over, but not fully engage, all instinct told me it was my Fuel Pump, so I had it towed to the dealership where the service manager was first telling me I got a bad batch of gas, now he is convinced it is the O2 sensor, now, I am just a girl, but I do know a thing or 2 about cars, and correct me if I am wrong, but I did not think that a faulty O2 sensor would kill the engine completely, just wear on the fuel economy and make the engine sputter a bit. Please help - I do not want to pay for a part that 1. I do not need and 2. Will not fix the problem.