| 
 | 
you would need something like this: 
 
http://www.ebay.com/itm/321914510023 
 
for the power connection.  
 
To run the opi, i connected ground (black) and +5V SB (Pin9, purple) to this connector. 
My ATX supply delivers 2.5 A on standby, without even being switched on.  
This way the opi runs silently.  
 
via software, using a GPIO pin and a Transistor you can pull down PS_ON (Pin 14 or 16, green)  
to start the power supply and power LEDs or motors or such. 
 |   
 
 
 
 |