I did a quick search in the RC forums for "Optimus" (you might want to do the same). The statement I found that summarized everything else I found was:
On 01/02/01 17:57 ET, Daniel Tonks said...
"Optimus", "Realistic" and "Radio Shack" are basically
You haven't even given us a model number (that probably wouldn't help, but it might). So your Optimus stereo could be almost anything.
OFA remotes (such as the bargain URC-6012w at Walmart) have a big library of audio codes. Quite a lot of those are in the list of audio codes that OFA has seen in Optimus models. There is a good chance that one of those codes will operate your Optimus stereo. But finding the right code may take a while and there is some chance it isn't present.