Disconnect the wire from the oil pressure sending unit on the side of the block. Then with the engine off, and the key on, touch the wire to the block or something metal on the tractor that will get it a good ground. That should make the light come on and the buzzer work. If they do, your sending unit is bad. 

If they don't, then check the wire where it connects to the gauge cluster and the buzzer for a good connection. Could be the wire wiggled out of a connector. Of course, this is assuming that the gauge cluster is getting power ok, but if it wasn't then the temp and fuel gauges wouldn't be working either.