Using PWR_OK to Activate an ATX Power Supply Automatically
The way a PWR_OK works, or at least the ones I have worked with, is by signaling the stabilization of the power output with a certain tolerance. This is used for sequencing purposes (e.g. When 5V line is ready enable the 3.3V PS).

This means you won't be able to use PWR_OK unless PS_ON is high first.