I just need verification that the Overclocked R4 can be used once per opportunity rather than once per round. Once in attack and possibly multiple times in defense.

Yep, as long as it's during the Combat phase, you're able to receive a stress token (there's nothing to prevent you from being able to), and you're able to be assigned a focus token (nothing that I can think of to prevent that either), then it works every time that you spend a focus token (as opposed to removing it). So it can work multiple times when attacking, defending, or when an ability spends a focus token during the combat phase.

If it was meant to only be used once per round, it would begin with "Once per round, (etc)".

Your only limitation is you must have a focus token and the opportunity to spend it during the Combat phase.