Prior to buy a new PS for your Optiplex you could check if it can be repaired.
Ther is a fuse inside.
Disconnect the power connector from the mains and leave the PSU idle for enough time in order to allow the capacitors to discharge. Be carefull to respect this point.

Open the box

Inside there is a fuse soldered on the PC board near the main power cable entry.
The fuse is packed in a transparent or black heat shrinkable sleeve.
Check the fuse and replace it.
Test the PSU.
If the fuse blow again, that means that the main diode bridge is destroyed.
That diode bridge (vertical black rectangular component) is screwd in the PSU frame as a heat sink.
This diode bridge can be easily found if you decide to replace it.
Replace the bridge and the fuse and test the PSU. (be carefull !)
Reassemble the cover.
That usually resolves the issue with the PSU.
Hope it helps.