If disconnecting the battery doesn't reset it then you will need the code reader. I'm just saying that didn't work on some other cars I've worked on, not that you do need one. Try disconnecting the battery first.

If your not sure the mechanic is telling you the truth just go to another mechanic for a second opinion. Don't tell him you've had it checked once already just tell him the light came on and you need it checked. If the second mechanic also says it's the O2 sensor then you're good. If he says it's something else, on to a third mechanic...