plz help 95 325i wont start

[bmw3forme]

i have a 95 325i, cranks over, but doenst start. its not trying to start. just turns over. car was running fine. check engine light had come on. took it to a mechanic that has obd1, said it needed an o2 sensor, as soon as i drove it out of his driveway. it died. and just turns over. i believe this car has a chain not a belt. it didnt have any noise, theres no clanking while turning over. Was thinking sensor. crankshaft or camshaft?? anybody experience this. havent checked the fuel pump yet. just want an input of probable causes. thanks for any help.