With the tank off, the light still illuminates...it's like the presence of the tank somehow eliminates the early ground. Weird, right?
To start off, put the multimeter in DC Volts mode, clip the black lead onto a solid, clean unpainted bolt on the engine case or the (-) terminal on the battery, slide the plug that connects to the top of the oil pressure switch upward a bit so you can make good contact there with the red lead.with the key switch on, check for voltage (should be 0 or something ve small) .Then start up bike and observe the light.MAybe have someone help you with either reading the meter, holding the lead on or wiggling the tank.
So because the switch is so close to ground, voltage drop should have already happened mostly and that's why voltage should be 0 or close to 0? Then, when the bike is turned on, the pressure switch should engage and break the circuit, causing the volt meter to read no voltage. However, if the light stays lit, are you saying that I should work my way up from the pressure switch testing contacts and trying to find a spot where there is a voltage? Then from that spot where I get a voltage (small) reading, from there to the switch is probably where the short is occurring.
I think that makes sense 
In normal operation:
The oil pressure light filament has ~ 12V on one side with the keyswitch on. When the engine is NOT running, oil pressure is "0" and the oil pressure switch is closed, providing a path to ground and causing current to flow and the light to come on. The switch has some small amount of resistance, so even when it is closed, that voltage at the top of the switch terminal where the wire comes down from the instrument panel will still have a small amount of voltage, but it will be much closer to "0" than "12". With the engine running, and no faulty grounds and proper switch, etc. the oil pressure pushes on the oil pressure switch, causing it to break the circuits path to ground -> with no current flowing, the light goes out. At that point, the voltage at the terminal on top of the oil pressure switch should be quite close to 12V because there is no current flow, but the voltage at the bulb is still the 12V.
NOW, if there is a short in the wiring between the bulb and the pressure switch, you will read something less than 12V. But, it doesn't require a full 12V drop across the light to make it light - usually around 6-8V will do, so if you wiggle wires and see the voltage at the oil pressure switch terminal change you will know that you are getting close to the point/location.
Also look closely at the underside of the fuel tank as the guys mentioned earlier - if a wire is rubbing and shorting against the fuel tank instead of the frame or another wire, that will have to be found visually.
