i have a 96 cadillac and the emisson light came on right after my oil change, what does this mean?

You mean the check engine light? It means you need the codes pulled.

You could disconnect the negative battery cable and find the E.C.U. This item is also reffered to as the vehicles brain or also could be known as the cars computer. If you disconnect this computer after disconnecting the negative battery cable and leave them both disconnected for about 15 to 20 minutes. Then after leaving them disconnected for a while plug the computer back in and then reconnect the negative battery cable in this order and as long as you have no more engine problems, That light should be gone. This is how you reset the E.C.U. good luck to you