Yup, WSM is Work Shop Manual. If I had an electronic version, I'd send it to you, but I've got the paper copy.
There are a total of 5 safety switches shown on the OPC circuit, as well as the key switch, stop solenoid relay, stop solenoid, battery and starter motor. The wiring diagram also shows a controller relay, but the OPC system circuit diagram does not. There are the 2 seat switches, 2 PTO switches and the HST pedal switch.
Basically, the testing procedure is to disconnect the switch wiring from the switches and test the resistance of each switch. The WSM says that all of these switches should measure either 0 ohms of resistance or infinity, depending on whether the switch is pushed or not. Depending on the resolution of your ohmmeter, the switch will not actually read 0. Mine reads 0.1-0.2, which is normal, as there is always a small amount of internal resistance in this type of switch. If they read anything else, they are defective and should be replaced. I was able to check the HST pedal switch and the seat switches while they were in the tractor, but I had to remove both PTO switches from the tractor to be able to accurately test them.
The 12v out from the 5 amp fuse runs through all of the safety switches and into the OPC controller. After studying the wiring diagram further, it appears as though the controller relay could be a possible cause, as well, even though the OPC system circuit diagram doesn't show it. The controller relay coil side is fed by a 10 amp fuse from the fuse box. When the coil is energized, the switch closes and sends 12v from the 5 amp OPC circuit through to the OPC controller.
If the PTO switches don't fix the issue, I will test the controller relay before buying the new OPC controller. The test for the relay would be to energize the coil side and test resistance across the switch side. Resistance should be 0 ohms. If not, replacement of the relay would be in order.