Due to age of vehicle, most likely cause is the o2 sensor itself is faulty and needs replaced. It will be the front bank 1 sensor. Follow bank 1 exhaust manifold to about the y-pipe location the faulty sensor should be there. 1st sensor you run into is sensor 1. Only other possibility is the MAF sensor is not operating properly.
If it is your faulty oxygen sensor for bank 1, sensor 1. This would be on the cylinder #1 side of engine, which should be the passenger/right side for this car. Sensor one is the front sensor before the catalytic converter. I would make sure that the Mustang is running smoothly before you assume your oxygen sensor is bad. You could also have an exhaust leak, fuel injection problem, faulty MAF sensor or an air leak in the system. You may experience a bit of rough running with some of these other faults.