o-dark-thirty

By analogy with "o-three-thirty" (0330), "o-four-thirty" (0430), "o-five-thirty" (0530), and similar times as often expressed on the 24-hour clock—early morning hours that archetypally share the characteristics of a still-dark/nighttime sky and a distastefully early time of rising/arousal; the letter o in these terms comes from the /oʊ/-for-zero pronunciation, which (despite its conflated nature) is idiomatically widespread (just as it also is in "two-point-oh" for "2.0", for example); thus, it is not homologous to the o' in o'clock.

Wiktionary, CC BY-SA 4.0 · source