[Jacques]

Thank you, Raion.
After I set "PWR UP" jumper, I can use the O2, but the machine work as soon as I plug in the power cord. The PSU is OK. Please help me to fix.

So it kicks on as soon as you plug it in? Sorry I don't know the answer for that but it could be a hardware problem somewhere else.

I've seen o2's that have the PWR jumper set and as soon as the power cable is plugged in, its turns on but doesn't do much else.  I've had to power down the o2, remove the PWR jumper, and then plug it in, that's when it started working correctly.  I hope that is OP's case

The pci riser can be removed, but you need it if any programmes require the hardware address of the machine. The riser contains a little Dallas chip, software with licenses linked to this will require the pci riser to be installed in order to run

Regarding the psu, check the frontplane power on micro switch, they get dirty and can also fail. Also check the contacts between the psu and frontplane.