oxygen sensors

'97 Buick Skylark V6 3.1 Ltr

had some other work done and afterward the mechanic said the engine light(which I knew was on) was the oxygen sensor. i do my brakes, transmission filter etc... myself so this would be easy.

but, how many oxygen sensors does the vehicle have? i was at a parts site and they had a front and rear sensor. they looked the same. the Advance site did not distinguish between front and rear. so if there are two is it one oxygen sensor fits all?

if i am replacing should i just go ahead and replace both while i am at it?

thanks