When used in the Overworld or any other world with air, the Oxygen Collector will always be at 100% output (when provided with power). However, on space stations and the moon the Oxygen Collector requires leaves in addition to power in order to produce oxygen. Each leaf block in an 11x11x11 cube centered on the collector will contribute approximately .4625% to the output (requiring 217 blocks of leaves for 100% output). Multiple collectors can use the same leaf blocks with no reduction to output.
Planted wheat can also be used to provide oxygen to the collector. each block of crops provides .9% oxygen input, and they don't decay; making them more efficient than leaves.
Oxygen Collectors will slowly consume leaf blocks in the area while they are on. To disable this, open the Core.config file found under the "\config\Galacticraft" folder and set the B:"Disable Oxygen Collector Leaf Decay" field equal to 0.