While “O Captain! My Captain!” isn’t very typical of Whitman, with its use of rhyme and stricter use of meter, there are a few elements that make it unmistakably Whitman-ian. To be exact, there are three (count 'em) aspects of this poem that have Walt's fingerprints all over them.
Number one: apostrophes. No, we don't mean those punctuation marks that look like commas in zero gravity. We mean appeals to people, and other things, that aren't necessarily going to answer right back. Whitman is a big one for communication in his poems. The trouble is, that communication usually consists of his speaker appealing to a person or group of people that may or may not be listening.
For example, maybe you've seen this Levi's ad. Did ya know that the poem, "Pioneers! O Pioneers!," was one of Walt's? Well, now you do. Notice the appeal to the pioneers (you can read the text here). It should seem pretty familiar after reading "O Captain! My Captain!" Heck, even the titles are almost identical. Whitman does this kind of thing a lot in his poems. He usually announces that he's about to go off on an apostrophe with that letter O. For another example, you can check out "A Noiseless Patient Spider." There, the speaker starts off observing a spider, but it isn't long before he's off exclaiming, "O my soul," and having an animated (and one-way) chat with his own spirit.
Numero dos: "O Captain! My Captain!" has a pretty boisterous crowd in it, cheering on the safe arrival of the ship and the work that the dead captain has accomplished. Whitman really loves crowds, what with all their hub-bub and hustle-bustle and shenanigans. They are a source of fascination and energy to him. As he writes in "Crossing Brooklyn Ferry": "Crowds of men and women attired in the usual costumes! how curious you are to me!" (3). Whitman is enthralled by these commuters, just as his speaker is fixated on the revelry taking place on the shore in "O Captain! My Captain!" What's so great about crowds, you ask? Well, for Whitman, the answer has to do with his third calling card…
Third Up: America—Walt was all about it. He loved it, he mourned it, he explored it, and he wrote about it from nearly every conceivable angle. This goes a long way toward explaining the sheer length of some of Whitman's poems. We mean, have you checked out "Song of Myself" lately? No? Go ahead. We've got a few hours to kill here.… Okay, you're back? Nice white beard you got there. It kind of reminds us of Whitman himself.
Seriously, though, it seems at times that Whitman is trying to document every aspect of American life in his poetry: the people, their work, the land, the history, everything. He had a cosmic streak in him, for sure, and he wrote a great deal about the soul, God, and the universe. But he was also a current affairs nut, devoting much of his attention to the goings on of his country and especially around his native New York. Having witnessed the horrors of the Civil War as a nurse to wounded soldiers, Whitman wrote extensively about this period in American history, and "O Captain! My Captain!" is just one example of that. It's not even his only poem about Lincoln's death. Check out "When Lilacs Last in the Door-yard Bloom'd" for another Whitman-ian take on this tragedy.