Talk of oxygen, for example (3)
I believe the answer is:
gas
I believe this is a double definition.
'talk' is the first definition.
(gas can mean to chat)
'oxygen for example' is the second definition.
(oxygen is a kind of gas)
'of' is the link.
(Other definitions for gas that I've seen before include "could be embarrassing for American" , "Nitrogen, say" , "Talk profusely" , "eg Helium" , "rabbit" .)