i agree its not right i get more than that to a tank in my cossie round town, im not sure what your mpg should be
an 02 sensor is a sensor fitted before and after your catalytic converter on the exhaust that measures the emmisions before and after the cat to firstly make sure the cat is working and secondly to ensure the correct air fuel mixture is used on your engine.
if one or both of these sensors fail your car will tend to run in a safety mode which richens the fuelling up causing you to use more petrol
by getting codes read i mean go to a garage where they will plug your car into a computer and see if there are any faults stored in its memory