For a primary, you shouldn't worry. The CO2 created in the wort by the yeast is pushing the O2 out. As CO2 is heavier than O2, it tends to push it outside.
I'm primary fermenting 2.5 gallons of beer in a 8 gallons primary fermenter without ill-effect.
For a secondary fermentation, this would be different. You would need to minimize the headspace.