Turning on a PC with a Orange Pi
The relay is unnecessary. Let the PC motherboard manage the power-on signal to the power supply. If you just open the relay to power down, you'll "crash" the PC, and it will require more time to boot up next time, possibly requiring manual intervention.

Your OrangePi just needs to pulse the power-on button signal to the motherboard to power-up or power-down the PC. You can monitor the main 5V output of the power supply to determine which state it is in at any given moment. When you power down this way, the PC operating system will shut down in an orderly fashion and then shut off the power supply.