rough idle

can anyone help me with this problem, i have got a 2001 renault megane 1.6 and it drives well and has lots of power but at idle, it just feels very rough and idling about 600 - 900 rpm. now a mechanic put the computer on it and the oxygen sensor on the front was reading 64 then went up to 800 and went back down to 77. what i want to know is should that sensor be reading like that? to me that readings is way out too much and i think the sensor needs changing. so anyone that can help me would be good.
cheers

When you say the readings went from 64 to 800 and back to 77 was the mechanic throttling the car at the time? If the computer has told you its the oxygen sensor maybe wise to replace anyway.