Will someone tell me what the purpose of the Oxygen sensor is please?

It tells the ECU about the fuel mixture under different driving conditions so the ECU can adjust fuel mix for the best economy and power. (and low pollution)
Should come out looking light gray in colour. White = too lean, black = too rich.

hi all I have a tp that when warmed up starts to play up, lets say at 200rpm it starts to cut out but only when warm and never when cold always at or around 2000 rpm?