several things could cause it.
1- Your oil sender unit is defective or close being defective. Sender is a normally closed switch by action of an internal spring. Upon start up, it maintains a closed circuit hence oil light pressure illuminating. As soon as pump start pumping oil, then the oil pressure will opens the switch and then oil pressure light will go out indicating pressure. Now this is for only "idiot light". The sender could also have a variable resistor, calibrated to digital or analog(needle) that actually shows pressure. This one is more preferable but mainly used on some automobiles more so than tractors.
2- If your oil level is low and you are in an incline (up or down) then you oil pick up tube and screen might suck some air while pumping, causing the pump to cavitate a bit, not making pressure and illuminating the low pressure light. A partially clogged screen can cause that and to clean that engine pan needs to be removed.
3- There is a very low possibility that your gear driven oil pump is damaged or has some restriction. This is very seldom a root cause but there is a possibility.
4- Check for loose wiring at the sending unit and also check all the fuses. Also check the connection at the back of tachometer.
By the way, I had to change my oil sending unit once 15 years ago. It was stuck closed.
