Have you checked all the wiring to the gauge from the oil pressure sensor?If it got tangled in something when you pulled the head you could of pulled the wire or possible have broken it.
If all thats good you can check the gauge by hooking the wire going to the gauge to an electrical ground and then blipping the key to see if the gauge moves. If the gauge maxes out while the key is turned your sensor is bad. And if it doesn't move then you gauge is bad.