Do you have 2 oil pressure switches or 1 most use 2 different switches 1 for choke 1 for gauge/light. They can be behind the distributor or above the oil filter,check both location make sure there isn't 2 switches. How quickly does the choke fuse blow if it's almost instant you have a short to ground,easy to find. If it's slow I'd suspect a bad choke coil. Short to ground pull the wire off the choke check resistance to ground,zero or close change the choke coil. Okay if that wasn't it probe the wire you took off to ground is it open? If not disconnect at the pressure switch is it open now? If yes check both op switch leads to ground,both open, the wire from the switch to power has a short. 1 or both oil pressure leads zero resistance to ground,the switch has shorted. If you only have 1 switch post back testing will be confusing using the diresctions I gave you because there are 2 circuits in the switch if you're checking the gauge circuit this test will be inaccurate.
