The OPC on your mower is simple. It sounds like everything is working except for the transmission neutral safety switch. It only sounds like someone has disconnected it. There should be 2 wires together going to the top of the transmission. Look around and see if it is hanging there anywhere. It will have a black plastic connector on the end. It may have been zip tied to other wires to keep it from hanging down.
You can download the wiring schematics for your mower from Exmark's website. Just download the appropriate parts manual for your mower. It will have the wiring schematic in it.