I recently put a full racing beat exhaust on my 82 gs 12a and my OES light comes on after I've drove the car and next time I start it the lights on and stays on for about ten minutes

 

I dont think the RB exhaust even uses that sensor...

 

That sensor is attached to the floor pan on the passenger side of the car (basically under the seat) and measures the temp of the floor above what would have originally been the stock cat. It's possible that your exhaust is getting hot enough to trip it (unlikely if the stock cat didn't) or that the sensor itself is bad. It's kinda useless you could pull up the carpet and just remove it should be fine.