No surprises in Spanish there, as you may already know:
aire and
oxígeno, just like in English. I imagine you don't really need any etymological information on those

(?), and as for some interesting compounds/metaphors, well, I don't think any of those come to mind right now in relation to
oxígeno, which tends to be very transparent (wink, wink

), but there are a few for
aire which I particularly like:
•
castillos en el aire = basically
luchtkasteel
•
matarlas en el aire = literally, "to kill them in the air", and used to talk about someone who's very quick-witted or skillful, and who can thus easily reply to whatever might be thrown at them, or take care of a problem or situation, for example.
•
ofenderse del aire = literally, "to be offended by air", which, as you may have already guessed, means to be particularly fussy, touchy.
•
comer tortas de aire = Literally, "to eat air-filled tortas". You won't find this one in the DRAE (nor perhaps in any other Spanish dictionary), at least not in this form, because it does appear there under other guises, but I've heard it here in Mexico at least a couple of times and while the meaning is far from positive or pleasant, I found the expression itself rather funny. A
torta, here in Mexico, is something like this:
So, of course,
comer tortas de aire refers to eating very little or nothing at all, or in a less literal sense, to have very little or no money, like in the following example:
"A tu hermano dentro de poco no le va a quedar de otra más que comer tortas de aire si sigue gastando el dinero así."
"Your brother will soon have no choice but to live on thin air if he keeps spending money like that."