deznosnake

I have recently purchased my 2nd Jag, and my first XJR. She is a 2001 with 95k on the clock. Also on the clock is the engine management light, which has been illuminated since the purchase. According to my friend whom I purchase it off, the fella who did the MOT said it's an O2 sensor. In just after some advice, like, how many are there? Where's best to buy them from? Are they easy to do? I'm good with cars, but not this particular area! Any advice would be very much appreciated!! Thanks