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".
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.
"Nahh, we're taking off at o dark thirty. You won't be up yet."
A measurement of time in the United States Army. O'Dark Thirty is usual reserved for the period between midnite and sunrise, which always is the coldest,wettest,darkest and shittiest of the year. It is also, without exception, the time of day when no sane Human Being would be found awake and/or sturring.