Can someone tell me how to remove the power supply unit from a Onyx2?
Your Onyx2 Deskside uses the same power supply as an Origin 2000 compute module. There's a photo of a skinless Origin2000 in the O2K section of the Nekochan Hardware Gallery
Disconnect the power cable (the black cable located towards the bottom. The removable end connects to the power supply; the fixed end of that cable goes into the fan tray where it mates with the main power connection).
The power supply is the nearly square gray-painted device from which you unplugged the black power cable.
Looking at that photo you'll notice two vertical metal strips about an inch wide at either side of the power supply (they look chrome plated, as opposed to the gray painted finish of the power supply). Those two strips are spring clips, gently depress the protruding tabs at either end (of the spring clip) to release tension so you can lift the spring clip out of the slot.
Four captive machine screws hold the power supply in the chassis, two at the top and two at the bottom - unscrew those.
There's a metal strip that runs horizontally across the middle of the power supply - it's actually the handle for a cam-lever that releases the power supply from the chassis. Pull outward that handle to release the cam-latch.
You should now be able to extract the power supply, but use caution - it ain't exactly light weight, and there are (plastic) connectors on the backside that can easily be damaged (or displaced so they don't make contact the next time you insert the power supply and close the cam lever