Oxygen is reactive and combines with all sorts of other agents. All that extra oxygen is locked up.
But not in ways that plants of today normally release it. More plants would only get you a few tenths of a percentage of oxygen, as the consume the last remnants of carbon dioxide. Currently, the limiting factor on plants is carbon dioxide and water available to them. Oxygen would clearly have left the system, but not because of the lack of quantity of plants.
Meteor strikes? Bleedoff into space? Bacteria that feed off of oxidized metals becoming rarer? These might do it. "Fewer plants" would only make sense if there was a lot of unconverted CO2 in the air today.