O2 sensor?

I have a 1994 dodge colt 1.5 liter with 185,000 miles. I've never really had a single problem with it but last week my check engine light came on for the first time. During the last week it has come on and off intermittently, but I have noticed that 90% of the time it would come on if I restarted the car after the engine was already warm. I was thinking it had something to do with the car trying to change the fuel-air mixture for cold or warm start-ups? My mileage hasn't really changed, maybe 31 instead of 33, but I thought that might be from the extreme cold we've had. Any advice would be appreciated.