Car uses the o2 sensor for partial throttle gas mileage turning it off you tune the individual fuel trims rather than depending on the o2 sensor to do it automatically.

Did you have a check engine light for the o2 sensor? If so then they had to turn it off.

Can it effect your gas mileage, not really as long as it's tuned to 14.7:1 during cruise. If it wasn't then yes it will effect gas mileage and emmisions output

again a simple google search could solve all your problems not trying to be a dick but look up stoich a/f or o2 sensor. This took me 3 seconds

http://www.autofamille.com/stoichiom...af-ratio-meter

who tuned it?