Which version of that particular range of OptiPlex's do you have? There are 4 flavors(in terms of size) an MT, DT, SFF and USFF. So which one do you have?
If you're looking at the DT/SFF/USFF versions then they have proprietary PSU's of limited wattage so any after market parts choices would be slim to none.
Take a look inside the case, it's the only way to be certain and it's no big deal getting inside a PC case, even on a factory-built system. For safety, unplug from mains supply first.