oh dark thirty
Definition from Wiktionary, the free dictionary
From military time, in which hours before 10:00 am are prefixed with a zero, read as "oh", as in 0530, oh five-thirty, for 5:30 am.
- Some unspecified hour in the early morning (implies an unpleasant time to be awake)
- I had to get up at oh dark thirty to catch a plane.