[…] We helped ourselves
to these green islands like olives from a saucer,
munched on the pith, then spat their sucked stones on a plate,
like a melon's black seeds. Pro honoris causa,
but in whose honour did his head-wound graduate? (Chapter 5)

These lines are taken from a section of Omeros that follows Major Plunkett, a retired soldier in the Queen's Army who has settled in St. Lucia with his wife. Here he's thinking about just how bad his fellow Europeans were when they exploited these islands.

Colonialism is a big theme in Walcott's poem. Like other writers, he's really interested in showing how lands and peoples were used and abused by colonizers. These lines are written from the perspective of a European character, which shows that anyone is capable of understanding the ethics and morality behind colonialism. Plenty of Europeans rejected colonialism, so active participation in it cannot necessarily be written off with a simple, "Oh, everyone was doing it."

It's all about imagery here. Walcott uses an image of eating to get at what colonial exploitation actually meant. Colonizers "ate" their colonies (ew), sucking out their nutrient, in the same way that someone eats an olive or a slice of watermelon and then discards it. The poet squeezes some heavy thoughts about colonial exploitation—economic, political, social, cultural—into this one powerful image of someone eating.