Given the age I'd say the O2 sensor is likely either bad or is giving a false reading. My 1996 Volvo had a check engine light come on this past spring. Turns out the O2 sensor was causing the reading yet there was nothing actually wrong with it--it was just a false alarm likely caused by gunk that had accumulated over time.
Word to the wise: Make sure you give your car a freeway run or two every month to make sure it opens all the way. My mechanic told me to do this (I typically drive short distances) and I have made it a priority to drive at least 10 miles on a freeway (on one drive) per month since. If you don't do this, you will supposedly get false O2 sensor readings now and then.
Try disconnecting your battery and then have your OBD-II read again (an auto parts store can usually do this free of charge for you if you don't have a tester) if it comes on again.
Word to the wise: Make sure you give your car a freeway run or two every month to make sure it opens all the way. My mechanic told me to do this (I typically drive short distances) and I have made it a priority to drive at least 10 miles on a freeway (on one drive) per month since. If you don't do this, you will supposedly get false O2 sensor readings now and then.
Try disconnecting your battery and then have your OBD-II read again (an auto parts store can usually do this free of charge for you if you don't have a tester) if it comes on again.