I'd be surprised if it was the O2 sensor on a 99.

There are hundreds of reasons the light might be on. Just not
screwing down the gas cap tight will turn it on. If you don't have
an OBDII reader to read the codes see if the local auto supply store
will read them for you (takes about a minute). Then look up the
code to see what triggered it.

I purchased my own reader when my CE light came on (98 S70 T5M). It was
an obscure item related to the wastegate control. So far, I've loaned
the reader to a couple of co-workers so at least a few others are getting
some use out of it, too.