Fatefully, oxygen present in O- likely discharged (9)
I believe the answer is:
ominously
'fatefully oxygen present' is the definition.
I can't tell whether this definition defines the answer.
'o-likely discharged' is the wordplay.
'o' becomes 'ominous' (I can't explain this - if you can you should believe this answer much more).
'discharged' suggests removing the centre (the letters inside the word released or discharged).
'likely' with its centre removed is 'ly'.
'ominous'+'ly'='OMINOUSLY'
'in' is the link.
Can you help me to learn more?
(Other definitions for ominously that I've seen before include "Forebodingly" , "with menace" , "apprehension" .)