Military time designating an unspecified time after midnight but before sunrise. Normally used in reference to either the time one goes to sleep, wakes up, or has to be on-the-job. Usually quite a while before sunrise, i.e.: well before 0500 hours ( 5AM for you civilians )

The name is a play on the way 24-hour time is handled in the Military. Times are pronounced as numbers and written without colons, i.e.: 4:30PM is 1630H or "sixteen-thirty hours". Times between midnight and noon such as 4:00AM are written as 0400 and have the leading zero called out as "zero" or the letter "O", as in "o-four-hundred".
Example: We had to get up at o-dark-thirty to get the job done this morning.
by Princess Die November 30, 2005
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Early in the morning before the sun is up.
Used to define when you must get up and go somewhere in the early hours of the morning between 1:30am and 5:30am.

a time in the am around the half hour mark.
"You're leaving tomarow? Will I get a chance to see you?"
"Nahh, we're taking off at o dark thirty. You won't be up yet."
by Cloudyjuicebox April 28, 2006
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Very early. Early enough to seriously fuck up the day before, and the day of the event in question.
Yo...we had to get up at O dark -thirty to get to the meeting on time.
by Rick Burns December 20, 2006
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