O say can you see … that this line begins with an "O" and not an "Oh"?
"O" may seem like just an old fashioned way to write "Oh," but it actually has a slightly different meaning. Consider some other famous O's: O Captain, my captain, O Pioneers, O Come All Ye Faithful, O Canada, O Brother Where Art Thou, O ye of little faith, O Christmas Tree. These are all examples of what's known as the vocative O — it indicates that someone or something is being directly addressed. When you say "O Christmas tree" the "O" means you are talking right to the Christmas tree. The rest of the song bears this out. (Your branches are lovely! You're always wearing that dress of green!) Same for "O Canada" and pretty much any anthem. The words to your school song probably go something like "O [alma mater], your campus is beautiful, and we think you're great."
"Oh" has a wider range. It can indicate pain, surprise, disappointment, or really any emotional state. While "oh, man!" could mean a number of things, "O man!" means "hey, you there … you man over there."
The convention now is that while "oh" can be lower case, and is usually followed by a comma, "O" is always uppercase and without a comma. But there hasn't always been a strict separation between the two forms. "Oh" and "O" were used interchangeably for a long time. The meanings often overlap too. When Juliet says, "O Romeo, Romeo, wherefore art thou Romeo?" is she addressing him in her imagination or sighing with emotion? A little of both. It's not hard to see why it's so difficult to keep a firm border around vocative O in English. These days it looks pretty rarefied and archaic and is usually only seen in old poem and song titles...
...ya, rly. Thanks to LOLspeak, "O" has found a way to adapt and survive, but there it's not limited to the vocative sense. The versatile, nimble "O" rolls on its merry way, picking up new meanings, and trying to stay relevant however it can.