This poem really rolls off the tongue, and that's no accident. It's designed to mimic the patterns of spoken speech. Basically, the whole thing reads like a one-sided conversation, in which the speaker has come to a decision to spill his guts to the addressee of the poem. He steels his nerve, takes a deep breath, and then he's off.
What lines give us this sense of verbal momentum? Well, pretty much all of 'em, but let's check out a few examples:
Check out line 8: "as solemn as unpleasantly definitive as statuary when right in front of it." Here the speaker is telling us how bummed out he is by statues (compared to his love for the you of this poem). What's his beef, exactly? The statues are "solemn." They're also "unpleasantly definitive." These are two, separate descriptions that get jammed together, with no punctuation, into the same line. It's as though our speaker is casually tossing out ideas as they come to him, not planning or punctuating for emphasis. Even in this line, which is an enjambed continuation of a thought from the previous line, the speaker is off and running onto a new thought before the line is even through: "when right in front of it." "When right in front of"… what? We have to read on to the next enjambed line to figure that out.
Enjambment and compression are two ways that O'Hara keeps us hustlin' though his poem. As we are sped along as readers, the true intensity of the speaker's conversation emerges. Consider the last line of the poem: "which is not going to go wasted on me which is why I am telling you about it" (25). Again, we have a line that carries forth an idea from the line above it, and again we have a line that does away with the traditional boundary-markers of grammar (commas, periods, and the like). Instead of two separate ideas—"which is not going to go wasted on me," and "which is why I am telling you about it"—we get both at once together, with no stopping or pause. Even to the end, the speaker is rushing to get this off his chest—not in a hectic way, but in a way that tells us as readers how important this realization is to him.