[dsraven]

unless they started building gauges differently these days I am pretty sure you need some continuity between the B and the T.
there are 2 coils inside the gauge making a magnetic field. one coil is a constant and it's field will move the needle all the way over to zero. the other coil has a varied by the amount of ground given by the sending unit. that field varies so the needle will more easily pull toward the zero mark by the stronger constant magnetic field on the other side of the needle. should the oil pressure spike then the needle would, of course, be drawn over to the high end of the gauge. it would make sense, then, that there should be some continuity between B and T. the internal coil would ground through the ground connection on the back of the gauge. since you have the gauge out you could easily test it on the bench with a battery and some test leads with alligator clips on the ends of the leads. connect batt neg to the ground, batt pos to the B and then see what the gauge does. it should spike to the zero end of the gauge. now ground the T connection with another test lead and the gauge should go to max oil pressure.
we are talking about some pretty small coils here so a continuity test may result in some pretty high ohms. sometimes best to simply test it manually with the old battery trick. you can also connect the oil pressure sensor to the air line if you have a compressor and use a pressure regulator to control the air pressure (with a gauge so you know what pressure you are feeding the sensor). the oil pressure gauge should react like normal. if it has nothing then suddenly as you increase the air pressure the oil gauge goes to full pressure you possibly have a sensor for an idiot light that is actually an on/off switch instead of a variable resistance sensor.