Openings for medics to inhale O2 (5)
I believe the answer is:
doors
'openings' is the definition.
(I've seen this before)
'medics to inhale o2' is the wordplay.
'medics' becomes 'drs' (abbreviation for doctors).
'to inhale' is an insertion indicator.
'o2' becomes 'oo' (Two o's).
'drs' placed around 'oo' is 'DOORS'.
'for' is the link.
(Other definitions for doors that I've seen before include "Barriers to entrances" , "Portals" , "Swinging or sliding barriers" , "Openings" , "Ways in and out" .)