Yes, the oil pressure switch should be where the bolt is located. The low oil pressure light is powered a white unfused ignition on power wire and from the light a white with yellow tracer or white with brown tracer wire goes to the switch.
I think on your truck the fusebox is under the bonnet. There should be a green wire from the fusebox running to the instrument voltage stabilizer, typically located on the rear of the speedometer. There are two male spade connectors on the stabilizer, and the green wire from the fusebox connects to one of these spades. It is likely the green wire for the oil pressure gauge was originally connected to the other male spade on the stabilizer. The two female spade connectors on ths stabilizer are the outputs and supply the nominal 10v to the temp and fuel gauges. Note that some aftermarket stabilizers have a male and female spade on each connection point, but the originals have male spades on the input and female spades on the output. The stabilizer looks like this:
Bob