Replace the sender, it's easy and cheap. Also check the wiring that connects to the sensor, because that might have got damaged in the course of the transmission swap.

I'm assuming that you didn't have a sudden oil pump failure upon first startup after the car was just sitting there waiting for a new transmission, so the only other thing I would check is a clogged up screen on the oil pump pickup.

Really though, after you've checked the condition of the wiring, and you've replaced the sending unit, and the problem isn't fixed, take it to the nearest shop (tow it?) and get the oil pressure tested (or get someone to come to you). If you truly do have a mechanical problem such as a worn/broken pump or a bad bearing, you are risking a major engine failure by driving the car.