There are many variations on this phrase, O’dark thirty, O’dark hundred, and O’dark. These all mean the same thing, early in the morning. Sailors do have to stand watch from midnight to 4am, and 4am to 8 am, so this saying does not normally mean this. However when a sailor must get up before reveille at 6am, it is early. While at sea sailors work and they work hard. So when they have time to sleep they relish this time and having to get up early for something is an inconvenience.

This getting up early can be anything from repairing a piece of machinery that has broken or because a higher ranking person thinks you need to get up and do something. This last reason is usually a mindless reason, just to upset the person who is getting woken up.

But when its time to put your boots on the deck for a serious problem, people will react and you would never know they just pulled 24 hours and only got an hour of rack time before they were awoken.

Yes, sailors might complain, but they know deep down, their job is important. So when there is a phone call at the White House at 3am, they know they will be called on to react