Originally Posted by
DaveReidUK
It's the tools and equipment used to work on an oxygen system that require to be clean and free from contamination by oil, grease, etc.
Every mechanic learns that as part of their training.
Indeed, but surely the much bigger question is: as the whole "oxygen clean" idea seems to have escaped them, either:
this was the first time any of these 3 had ever worked on an oxygen system
... or ...
These guys had previously worked on other oxygen systems, using similar procedures, and there's a bunch of other boeings flying about in a similar condition but meh, no one is keen enough to sort out that mess.