On the 24-hour clock the hours before 10 am start with a 0; so 1:00 am is 0100 and said as oh-one-hundred and so forth. Thus oh-dark hundred is anytime after midnight while it is still dark: "They woke us up at oh-dark-hundred and ran us thru the obstacle course." means: they woke us up in the wee hours of the morning before daylight. More @Wikipedia
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