An Oxy-Generation Unit, simplified as an O.G. unit, was an essential component of spaceship life-support systems. The O.G. unit not only generated oxygen, but also maintained breathable air for the crew.
When the boys from the Dwarf were attacked by an Enforcement Orb and Starbug was forced to crash land on a swampy moon, the auto-repair system of the 'bug was able to fix almost all the damage. The only item that was broken beyond repair was the O.G. unit, which Rimmer described as "a black and charred mess, worse than one of Dave Lister's drunken fry-ups". Without the O.G. unit, the 'bug could fly but not provide enough life support to sustain Lister and the Cat.
Their only option was to try trading with the local repulsive GELFs. The Dwarfers found a Kinitawowi village and offered them many goods in trade for the O.G. unit which the Kinitawowi had acquired from somewhere. The trinkets provided by the Dwarfers granted entrance into the hut of the Kinitawowi chief, or watunga, where the bartering began proper.
The GELFs brought out a portable O.G. unit contained in a a large, silvery metallic briefcase. However, the Kinitawowi chief declared that he wanted Lister to marry his daughter, Ech-Ech-Ech-Ech-Ech-Ech-Ech-Ech, in exchange for the O.G. unit. Lister went through with the ceremony, and his crewmates left with the briefcase. However, before consummating his marriage, Lister ran off to join his crewmates shouting "change of plan! LEG IT!".
The replacement O.G. Unit was installed by Lister and Kryten, while Starbug took off using its single hours worth of emergency oxygen supply. The Dwarfers were unaware that the Kinitawowi had sent their Emohawk to attack the crew in revenge for reneging on the deal and stealing the O.G. unit. (Emohawk, Series VI)