O Captain! my Captain! our fearful trip is done (1)
In poetry, the “O” is usually used in poems that appeal to whatever follows the word. The speaker has shared experiences with the captain that allow him to take possession of the captain by using the word “my.” Also, the poet has chosen to capitalize the “C” in “captain” to make the man seem even more important, both to him and to us as readers.
The ship has weather’d every rack, the prize we sought is won; (2)
Notice how the prize is whatever you the reader want to imagine, and how everyone would be afraid of getting caught in a big storm out in the ocean, Perfect Storm-style. The dangers and goals that the captain has guided the speaker/crew through are so vague that every reader can sympathize. As a result, the captain becomes even more readily admirable.
The port is near, the bells I hear, the people all exulting, (3)
To exult is a high form of celebration, though the reader is not told exactly what the people are really happy about. It could be the return of the ship, the successful captain, the success in winning the prize, or D) all of the above.