If you just started your car, the oxygen sensor is cold, and will not give any reading for a little while. This can also happen if the car has been idling for a long period of time.
Also, if you have removed the lower honeycomb from your MAS, your oxygen sensor reading at idle will likely drop to zero (or almost zero) at idle. This is a common side-effect of removing the lower honeycomb, and does not represent a problem. This effect is only affects O2 sensor readings at idle, and will not change the sensor or car behavior while cruising or while accelerating.
For more information on how the ECU handles fuel, read the The Essential Primer on the DSM ECU.