The closer to the turbo the less it's life will be, it ideally needs to be at least 1 METRE away from the bearest exhaust valve, widebands will overheat under boost, you must also have at least 20" after the lambda of exhaust left as well.
This applies to a cossie, unless normally aspirated where different criteria can suit and generally the lambda can be closer.
Ideally mounted between 10 degrees from the horizontal to 15 degrees to the vertical, due to heat soak.
hot cams that give lots of valve overlap can give very lean readings at idle and low power due to air escaping into the exhaust at "off cam" operation.
It differs a bit if the sensor is a PUMP CELL sensor like BOSCH 4.0/4.2(same thing!!)
The heater in the sensor can warm the sensor no problem, but it cannot cool where they are placed too near a red hot turbo.
You could even go for a tail pipe sniffer design, but not needed on a cossie.
Depending on the controller, these will tell you whether it is too hot or too cold, Tech Edge have this facility.
tabetha
info gained from tech edge website, compiled with the help of BOSCH