Volvo: O2 Sensor replacement, computer control system, o2 sensors

Question
My mother has a 2000 s80 twin turbo that needs the o2 sensors replaced. the code is P0030. She bought 2 bosch sensors because the servive guy said she HAD to replace "Both" sensors. the printout says "HO2S bank 1 sensor 1 heater circuit." which sensors would they be? her car has a total of ... sensors that i see.
               P.S.    i don't know much about volvos. i'm usually working on my bronco. plus my knowledge is limited to only what i have tought myself by "tinkering" in the last 4 years.

Answer
Adam,
I would bring back the sensor that were bough and let the dealer get them. I have heard of many problems with O2 sensors not being the correct ones for the car. Volvo has updated them. The computer control system is real sensitive and need the right sensor to work.
It maybe best to replace all four as they all are as old and showing wear.