Postmodernism, Epic Poetry
It's pretty clear from the title and the length of the work that we're heading into an epic poem. But what does it really mean to be "epic"? You know—when you're not using the word to describe a party.
In the tradition of Homer and other epic dudes like the Beowulf poet, Virgil, and Dante, such a poem relates the heroic journeys of a person or group. They move through treacherous situations, encounter the supernatural, do superhuman deeds—but despite their adventures, the language stays formal and lyrical, using a rigid metrical form to make the verses easily memorable for recitation (think about the griot in Achille's dream-vision who sings about the past).
Walcott shows a lot of love for the epic form, but he also has his own ideas about what an epic can be and do. His poem is about fishermen, not kings, which is a major departure from the ancient form. He's also interested in deconstructing the traditional narrative, inserting himself as narrator in non-traditional ways (recall his attendance at Maud's funeral) and focusing on social/historical ills rather than the heroic deeds of a privileged class. He also pays homage to James Joyce, a postmodernist idol. All of this places Omeros comfortably in postmodernism.
Good luck finding the postmodernist epic poetry section at your local library, though. Might be good to ask for help finding this title.