Oscar, then out to lunch, meets Dorothy punctually (2,3,3)
I believe the answer is:
on the dot
'punctually' is the definition.
(eg 7 o'clock on the dot means 7 o'clock exactly)
'oscar then out to lunch meets dorothy' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'out to lunch' is an anagram indicator (out to lunch can mean mad).
'meets' is a charade indicator (letters next to each other).
'dorothy' becomes 'dot' (short for Dorothy).
'then' with letters rearranged gives 'nthe'.
'o'+'nthe'+'dot'='ON THE DOT'
(Other definitions for on the dot that I've seen before include "Precisely punctual" , "At exactly the right time" , "at exactly the arranged time" , "At precisely the right time" , "Absolutely punctual" .)