The "check engine" light came on Monday, and I finally broke down and bought a scan tool. It's telling me the problem is "P0133, O2 sensor circuit slow response."

One of the parts house staff is telling me there could be multiple causes of this, and not to buy the oxygen sensor right away. Can this be so? What else could cause slow response? I'd rather not spend $75 or more on an oxy sensor if I don't need to, but hate not to fix the problem, since it renders the "check engine" light pointless if it's on all the time.