You're right that its the first O2 sensor, but not for the reasoning you gave. Because all MIL codes are universal, it will give you a "Bank 1, Bank 2" reading for which side of a V engine the problem resides (Bank 3 and 4 for a W style engine).
Sensor 1 (sensor 2) indicate which one it is. The numbering goes downstream from the engine. If you're unsure which sensor it is, if you look down in your engine bay at the heat-shield covering your exhaust manifold, you'll see a plug that looks like a spark plug sticking through a hole near the bottom of the shield with 2 (or is it 3?) wires coming out of it. That's the #1 O2 sensor. FYI, it can be a b**** to get out since it's subject to the full blast of heat from the engine...
If you need any more help, don't hesitate to ask. Good luck!
