As far as the wires, one wire gets hot when the key is in the on position, one gets (should) hot while the starter cranks (provided the starter was wired up with the wire on the correct studs on the starter solenoid) , and one goes to the + side of the coil (and + side of fuel lock-off). this is a switch, not so much a sender (not a resister to work a gauge). it prevents the engine from running should it ever loose all that oil. usually there is a tee fitting and a separate 1 wire sender for a gauge if it was equipped with a gauge.
you can use an ohm meter to see which connections are a closed circuit at rest then at pressure to see where the wires go on the switch. One should be common to both conditions, it is the one to the coil circuit. the wire that goes hot with the key switch 'on' goes to the one that makes continuity with the common circuit when the switch has pressure, and the one from the side of the starter that gets hot only when the starter turns, goes to the one that has continuity with the common screw when there is no pressure.
my "instant wild as guess" [WAG] would be the wire that should be connected to the starter solenoid is touching the starter or motor and grounding out, and/or is not on the correct post for the small wires (there are 2 small wire posts, one comes from the key switch 'start' side, and pulls in the starter solenoid, and the other has power while the solenoid is pulled in, you should have the wire to the oil pressure switch on the one that gets power -only- _while_ the solenoid is engaged.

I need to know how many wire's go to oil switch because there are 3 screws and where those wire's come from

the 1404 is a Caterpillar (not 'Mitsubishi') motor designation, sometimes referred to as a "Hercules" motor in other brand of forklifts [some Daewoos had the same setup].
It is a pretty simple wire harness and does not have much in the way of bells and whistles. does the fuse blow as soon as you turn on the key" or as soon as you put it in gear? or as soon as you turn on the lights?
Have you installed any lights or other components that run wires up the overhead guard? have you taken a 'good eyeball' to the wires where they go into the distributor?. Those are the first places I would be checking first.

Need wiring diagram for oil switch and ignition

It is a Mitsubishi 1404 engine. Just found out that fuse number 1 which is 10a is blowing when replacing it and turning ignition key on it blows right away? Any idea what could cause this?

Thank you I will check that.

if you have no 12v at coil + when key is on then you have break in wire from keyswitch to coil.

I would still check the engine oil pressure switch and see how many wires going to it, if more than 2 then check the switch and wiring, sometimes they have ignition tied into oil pressure switch to kill engine if oil pressure drops, if switch is bad then engine will not start.

Also open the panel on the front of the steering column and check the fuses in the fuse block there, they are bad to corrode up and have bad connections.

It is a cat 4cylinder propane fuel it was driving then just stopped now no spark nothing @ coil or no 12V to wire to coil when key is on or cranking.

what engine is in it?
peugot 2.0 XN1P?
or cat 1404?

give us a serial number on lift dataplate if u can.

my guess though is maybe an engine oil pressure switch not working
or vacuum safety switch, worse case scenario might be a bad ignition module.
it all depends on what system is on this truck