2001 A6 Avant Quattro 2.8 ATQ

I got these two codes (P0130 & P0150: O2 Sensor Circ.,Bank 1-Sensor 1 Malfunction & Bank 2-Sensor 1 Malfunction) as well as P0420 Catalyst Sys Bank 1 Eff below T-hold & and an addtional code not listed in the book (VAG #16814: I assume is Catalyst Sys Bank 2 Eff below T-hold). Ok, how to determine if it is the O2 Sensor's or the Cat Converter? Then, Bank 1 vs Bank 2, Sensor 1 vs Sensor 2(Left & Right or Front & Rear)? Can someone provide some insite. I'm confused.

And one more code: P0442 Evap Emiss. Contr. (small leak) Leak Detected. Think this is related to the above?

I'm pretty good at mechanical details but not so much at emission/electronic diagnostic.

Any help is greatly appreciated.