Stories used as oxygen, say (5)
I believe the answer is:
sagas
'stories' is the definition.
(saga is a kind of story)
'used as oxygen say' is the wordplay.
'used' indicates anagramming the letters.
'oxygen say' becomes 'gas' (oxygen is a kind of gas).
'as' anagrammed gives 'sa'.
'sa'+'gas'='SAGAS'
(Other definitions for sagas that I've seen before include "Heroic tales" , "Myths" , "Long involved stories" , "Tales of heroes" .)