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".

"It's oh-six-hundred" - he's put a double-meaning into this military phrase for time (0600hrs). "It's O(h my God!) six hundred!" (meaning "too early!!")