O2 filling

A bit O.T. :
I read an accident report a few years back - an FBO in Europe re-filled an
O2 bottle with compressed air. In the ensuing high-altitude flight, the
pilot lost conciousness and the airplane crashed in the mountains. A
passenger, miraculously, survived and confirmed the cause of the crash due
to the pilot's unconciousness.

What I've wondered ever since I read it :
What would the FBO be doing with compressed air in the first place, and how
could this confusion occur? What *could* they have been thinking?