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.
Initially the earth's atmosphere had less than 1% oxygen. But activity by blue-green algae species billions of years ago gradually increased these levels. For them it was just a waste product of a respiration process that relied on releasing hydrogen from water. By 1.3 billion years ago, levels had risen to 1%. Around 500 million years ago, oxygen levels had reached 10%. This was sufficient to switch on the all-important ozone layer, which protects the earth's surface from blazing destructive UV radiation.
From then on, other life forms could evolve.
O2 may have reached 35% in the late Carboniferous age, when life was mainly plant-based. The present level of 21% was settled around 5 million years ago. As a result, oxygen is the most prevalent element in the Earth's crust (53%); rock is basically silicon dioxide, with additions
A lot of it appears to be bound up in the vast hydrocarbon deposits, such as coal, that came from the period of high oxygen (the Carboniferous).
This Word document seems to indicate that some of that extra oxygen now comprises the ozone layer: http://trc.ucdavis.edu/ATM_ECI149/Notes/ChI-04-2.doc