Im not too confident in your tuner, unless you have misunderstood him.
The o2 feedback has to be switched off when tuning a Nistune board, but then turned back on afterwards.
As the o2 sensor is a narrow band, it can only show when the car is at 14.7:1. When you blip the throttle it would go richer than this, so of course the sensor will show deviation from 14.7:1.