My engine light is on and i was told by the mechanic that the oxygen sensor is dead. How critical is getting that done.

Secondly, my left wheel is making a "tuck, tuck tuck" kindda noise. I was told that i need to replace the mount and the bearings for the strut on the left wheel. He suggested that i should replace it on both sides in the front, but said its up to me as it should be fine if i just replace the left one.

Thanks