Yes you can certainly use a psu from 3080/5080/7080 MT/SFF in 5090 SFF.  they all have two white 4 pin cpu connectors.  Form factor is interchangeable in optiplex series 6 and 5. the psu from 5050/5060/5070 etc. (series 5) etc. has only one 4 pin cpu. the proprietary white 6 pin connector is ubiquitous in recent Dell models. Re: 300w psu in back order.  You can also use psu for XPS 8940 or Inspiron G5 5000.  They are compatible w 5090. [IMAGE]  
Hi Everyone!   Long story short, I have an optiplex 5090 SFF Pc and want to upgrade the PSU. The Dell sales team says the 300W is on back order.    My question is, can I use a PSU from a different optiplex series if it fits?   Thanks!
Can I Use a PSU from a Different Optiplex series? Looking to upgrade!