Sensor has to be deep in the tailpipe to get it away from any free air pulses.
As long as it's in the dirty air of the exhaust, preferably closer to the engine, it will read O2 content in stream. Check out where sensors are in new cars. Front ones are anywhere before the cat. If placement made a difference, the OEMs would be all over it. Collector, pipe right after reducer, won't make any difference with that placement.