There are 4 sensors, though not all of them are technically called "oxygen sensors. There are two that are upstream of the catalytic converters, one on each bank. Then, there are two more, one after each catalytic converter, that are oxygen sensors, but officially they are called catalyst monitors, because that is the job they are assigned to do. To recap, the two before the cats (or upstream), are oxygen sensors, and the two after the cats (or downstream) are catalyst moniters. Although all 4 of them are oxygen sensors, their titles are different.