So, as Dave said, that is the only difference, Percodan contains Aspirin and Oxycodone and Percocet contains Acetaminophen and Oxycodone.
For some people, the Aspirin works better for them with the Oxy to relieve their pain, and for some people the Acetaminophen. Sometimes it also depends on other body conditions, like liver or kidney function or problems, so that can have an effect on what the doc prescribes. There are also some new formulations that use Ibuprofen.
My information is not guaranteed correct. I do not get them right all the time, but I do enjoy the hunt~