oh dark

(Usually military) Slang term used to describe a very early hour in the morning.

We may as well catch some sleep this afternoon, 'cause it's wheels up at oh dark thirty.

oh dark 30

05:30 in military time is pronounced
Oh five thirty

Oh dark thirty (o dark 30) means usually getting up at some unreasonable, unnatural, ungodly early time such as 2am when you usually get up at 6:30.

It does not really mean half passed the hour. Just a early time.

To beat the traffic for our trip, we need to get up at oh dark 30.