As well as Adama and Roslin on the planet (Boxing Episode Flashback).
Ah, but the key element (in this discussion) is "on spacecraft".
I think the idea is to suggest that the technology is so sophisticated and space travel is so commonplace that the problem of using up valuable oxygen becomes trivial, as opposed to a kind of space travel where the resources are pushed to the limits in experimental craft.
I've seen a number of relatively recent movies or shows (not in futuristic settings) where they'll pointedly have, say, a doctor lighting up a cigarette, or a Suburban Dad with a pipe, in order to give it a '50s look. More likely, it's intended to be a bit of a satire of '50s shows.