What year is your SVX? If it's pre-96, you can run the check engine diagnostics yourself. It's very simple. Look at the "Codes" section here. Otherwise, Autozone will check the codes on your '96 or '97 for free.

And since he told you it was "either the knock or O2," I would think that he either did not actually run the diagnostic or just didn't feel like working that day. The diagnostics will tell you exactly that threw the code. It would say "O2 - left side," or "Knock sensor - right side."

If it truly was "either the O2 or knock," the problem may very well have been the knock sensors. That would explain why the light is still on. I would suggest that you somehow get the codes on your own terms — either do it yourself, or have Autozone do it. Keep in mind that there are two knock sensors on all SVXen, two O2 sensors on all pre-96 models, and three O2 sensors on all '96 and '97 models.