I am going by memory here so...it certainly would be easier if you would tell us Year and model. Then you could refer to the manuals and wiring diagrams here.
I see this on many forums be the ones for Vespas right up to the ones for D10 Cat bulldozers! Posters forget that not everyone knows what piece of equipment everyone has out in the shed.
One good example of something to do is like Petew does right down at the bottom of his post it shows what Yamahas he has. So if He were to say I'm having a problem with my `78 you know right off it is a 1978 XS400-2E and not a 1978 Dodge pick-up truck or something else!
I'm far from an expert on the wiring of an XS400 but the oil pressure switch should not cause a no start condition in any machine I know of. It is a simple on/off switch that turns on the oil pressure light when pressure is low and as soon as you have some set amount of oil pressure it turns the light off.
If I was thinking I may have shorted something out testing wires the first thing I would be checking would be fuses. They some times make a pop noise when they blow.
Now it is possible that if you shorted the right wires you could have sent voltage down to the oil pressure switch and as it is a normally closed switch that grounds one side of the oil pressure light to turn it on it may have made a popping when it burnt up and cracked the insulator causing it to leak.
Basically I think you have two problems, bad/leaking switch caused by sending voltage to a grounded switch and then another problem causing the no start. As for the no start the first question I would have is does the bike crank over, assuming it has an electric start, another good reason to give more details about a bike when asking for help!
