I had the oxygen sensor replaced in my 2002 Nissan Altima two days ago. The service engine soon light still has not went off. Does anybody know how to reset it or if it will reset on its own?

It will reset on its own, but it'll take a while.

Generally speaking, codes, once repairs are complete, will clear after 40 drive cycles.
A drive cycle is usually defined as a start up, then warm up, then cool down. So you can see, it'll take a while.

Find a parts store, like AutoZone or the like, and they can turn the light off more quick.

Try removing the negative battery cable for 15 seconds.