I criticised keeping oxygen separated (8)
I believe the answer is:
isolated
'separated' is the definition.
(I know that separated can be written as isolated)
'i criticised keeping oxygen' is the wordplay.
'criticised' becomes 'slated' (I've seen this before).
'keeping' indicates putting letters inside (some letters kept inside).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'slated' going around 'o' is 'solated'.
'i'+'solated'='ISOLATED'
(Other definitions for isolated that I've seen before include "Segregated" , "Quarantined, alone" , "Placed alone" , "Placed apart from others" , "Remote; lonely" .)