Well, it's "Oh-six-hundred (hours)/ O-six-hundred (hours)- hours is optional". It's the "24-hour time system", which is mainly use in military (So, also called 'Military Time'), and transport services like trains, buses and airplanes.
When someone says, "Oh/O-six-hundred (hours)", they means "0600", or simply, "06:00 a.m./ 06:00 o'clock in the morning".
And now your question- What does "O" mean here ?
Well, the answer is that either "O" or "Oh" is said for "Zero" or "Naught".