Cutting costs twice, Oscar, Mark and Mike – who live together? (9)
I believe the answer is:
roommates
'who live together?' is the definition.
'cutting costs twice oscar mark' is the wordplay.
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'costs' becomes 'rates' (rate is a kind of cost).
'twice' shows that letters should be duplicated.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'mark' becomes 'mm' (I can't explain this - if you can you should give a lot more credence to this answer).
'o' doubled is 'oo'.
'oo'+'mm'='oomm'
'rates' going around 'oomm' is 'ROOMMATES'.
'and mike' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for roommates that I've seen before include "Associates" , "Fellow lodgers" .)