Each collector extracts 1.5 air point per second from each leaf block. With this value in mind, you can calculate how many leaf blocks are needed within the range of the Oxygen Collector to supply sufficient oxygen for sealers and other machinery.
Place down the Oxygen Collector near some leaves and give it power. It will automatically start to suck the oxygen out of the leaves around. Note that the leaves will be consumed slowly over time, and will have to be replanted/replaced. The more leaves surrounding the Collector, the more oxygen it will collect. Alternatively, you can place it near growing wheat which will not be consumed.
If you are on The Overworld, simply place down the oxygen collector and it will immediately start collecting oxygen. In other environments without oxygen (such as the Moon, Mars, etc.), leaves will be required for it to work.
Connect the collector to a power source, Battery, Coal Generator, or (Buildcraft/Universal Electricity/IC2). You can then connect the Oxygen Collector to a Oxygen Bubble Distributor, Oxygen Sealer, or Oxygen Compressor using Oxygen Pipes. The bubble the Distributor creates is bigger depending on how much Oxygen is being collected. Multiple Oxygen Collectors can be hooked up to a Oxygen Bubble Distributor.
UE Crafting/Basic Components: