Have to reset check engine light?

My 1999 volvo has a check engine light. Had it checked and 'twas the O2 sensor. I verified it was not working and replaced it.

Question: Do I have to have the CE light reset OR will it go away after so many cycles? It's been about at least a dozen start/stop cycles by now. Will just disconnecting the battery for a short while clear it?

(Alternatively, I should just spend the $ for a code reader.)

Thank you.

Disconnecting the battery might clear the code. Try it first. If not, then you can either buy a code reader, or take it to a shop (bring the recipt for the O2 sensor) and they'll probably reset it for you.