If you let beer stand in the glass then the first thing that will change is temeperature.
Assuming the bottle is cold ( 8 or 10C ) and you are not in an igloo, room temp is about 20C
What happens?
The beer warms up!
Now as beer warms up solubility of CO2 decreases with temp so the beer outgases CO2 slowly.
Until there is no CO2 left there can be no disolution of atmospheric O2 as the beer has a dense head of CO2 to act as an inert micro-atmospher over the beer.
So I would suggest that the major flavour changer is temperature and not oxidation.
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Will