Bank 1 is the side of the engine that has the #1 cylinder.
Bank 2 is the opposite side.
Edit: In this instance, if you're going to replace one O2 sensor, you may as well replace both.
BTW, my apologies, OP. I answered the question but didn't really answer the question. My point is that people mistakenly think that Bank 1 and Bank 2 automatically refer to either the driver's or passenger side of the car, when it really has to do with which side the #1 cylinder is on.