Altogether it's the Oz dog (2,4)
I believe the answer is:
in toto
'altogether' is the definition.
(I've seen this before)
'the oz dog' is the wordplay.
'the oz' becomes 'in' (I am not sure about this - if you are sure you should believe this answer much more).
'dog' becomes 'toto' (I've seen this before).
'in'+'toto'='IN TOTO'
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for in toto that I've seen before include "overall" , "Completely - entirely (Latin)" , "All told" , "As a whole (Lat.)" , "Altogether, completely" .)