An O.G. unit was a grey box roughly the size of a suitcase. It was an interchangable component, easily replaced with another compatible unit. It could be installed in about an hour.
When Starbug was attacked by an enforcement orb and forced to crash land on a swampy moon, the ship's auto-repair system 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". Without it, the ship could fly but not provide enough life support to sustain Lister and The Cat.
Their only option was to try trading with the local Gelfs. They found a Kinitawowi village and offered them many goods in trade for the O.G. unit that the Gelfs had acquired from somewhere. However, the Gelf Chief wanted Lister to marry his daughter, Ech-Ech-Ech-Ech-Ech-Ech-Ech-Ech, in exchange for the unit. Lister went through with the ceremony, then ran off with his crewmates once they had made the trade.
The O.G. Unit was installed by Lister and Kryten while Starbug took off using its emergency oxygen supply. They were unaware that the Gelfs had sent their Emohawk to attack the crew in revenge for stealing the unit. (RD: Emohawk: Polymorph II)