In the fourteenth stanza of Part One, the speaker describes a snow-filled crevice that they sail past; it is said that it gives off a "dismal sheen." This is an oxymoron because dismal means gloomy and depressing, while sheen means shining or resplendent.
In Part Five, it is oxymoronic that "with his cruel bow he laid full low / the harmless Albatross." "Cruel" and "harmless" are opposites that are juxtaposed here to describe the fateful killing of the bird. In the next stanza, another example exists: "He loved the bird that loved the man / who shot him with his bow." The bird's love for his killer is an odd and wholly unexpected sentiment.
A third and final example of oxymoron appears in the poem's penultimate stanza in describing the mariner: "The Mariner, whose eye is bright, / Whose beard with age is hoar." Bright eyes are associated with the young and vital, which the mariner is not; hoar, however, means to be gray-haired or bearded with age, which of course the ancient mariner would be.