In some space flights they used oxygen at a low pressure so that it was as breathable as earth air.
Yeah, pre space shuttle US space program flights all used pure O2 pressurised at about 5psi, as the lower pressure meant less strain on the pressure compartment. This caused additional complications during ASTP (First Docking of US and Soviet spacecraft in 1975) as Soviet Craft used 14 PSI mix of Oxygen and Nitrogen. They went with this mix for the space shuttle as it is more like the earths atmosphere and so more suited to longer flights.
Not sure about the Space Shuttle but Russian space stations have employed forms of Oxygen Generating units, not sure of how they work exactly and Lithium Hydroxide cannisters for removing CO2 form the atmosphere.
Also, if anyone is interested; additional oxygen is carried in tanks to power spacecraft fuel cells. They burn Hyrogen and Oxygen to generate electricity for the ships systems and a byproduct is Water, which can be drunk by the crew.