I see wiring has already been mentioned. You can quickly rule out wiring with a multitester. Set it to read ohms. hook one lead up to a ground and put the other lead on the oil pressure sender. With the engine off there should be no resistance. With the engine running there should be infinite resistance. If you get these readings your oil pressure sensor is working and you have a wiring harness problem. If you get no resistance with the engine running either your sensor is bad or you really have no oil pressure. I would probably try a new sensor first, as they are pretty cheap, and low oil pressure is rare. I would also suggest that if your oil pressure really is low(unlikely) you would be better off getting another motor.
If you are not comfortable with working on cars you should get a mechanic to do this.