Gosh!
I'm not sure I like that clue .. it seems too self-referential or incomplete.
OTT = Over the Top = 'extravagant'.
O = an abbreviation for 'oil' ? or does the 'primarily' signify just the first letter of 'oil' ?
Then there's OTTO = an altenative spelling for 'attar', which is a fragrant essential oil.
JimC