location of bank1 O2 sensor?

under the hood or under the car, is it easily disconnested? GM 3100... 1998 monte carlo

It is before the cat where the rear mani and the crossover connect to the DP (downpipe) You can reach it from under the hood! But it will be a pain to get off. So use monkey piss and a box wrench, iirc it was a 20mm! They also make a socket for it but I have found them to be a pain.