the first thing to check is the actual pressure switch
put a new switch in there to eradicate te issue of the switch being at fault

then make sure you run a new wire to eliminate a possible break and and earth giving yo these sypmtoms

then stick in some regular oil, find that opie oils chap and see if there is a recommended oil for older engined cossies if you want some technical info on it

the ford oil pressure switch will come on at between 4 and 7psi and is more of a "your engine is fooked, get it sorted" rather than "your pressure isn't as high as it should be, get it looked at"

once you have done this and the light stays on then its time to pull the engine out

but you could be looking at an oil pump or a whole engine rebuild, but better not to run the engine for making it worse than carrying on and hoping for the best