The book uses an oxymoron to describe Doña Maria's difficult and lonely upbringing:
"...her mother persecuted her with sarcasms in an effort to arouse some social charms and forced her to go about the town in a veritable harness of jewels."
This passage is describing Doña Maria's mother's harsh attitude and the clothes she was forced to wear in public. The phrase "harness of jewels" may not seem like an oxymoron, since the words do not have inherently opposite meanings. But in the context of this passage, Wilder draws on the words' implicit connotations to create an oxymoron. A harness, a device worn by horses and other livestock animals, signifies bondage or servitude. Meanwhile, jewels represent Doña Maria and her family's wealth and class status. By describing Doña Maria's jewels, the symbol of her privilege, as a harness, the novel suggests that belonging to this class is a form of servitude for Doña Maria: because of her looks and personality, she is unable to fit in, has difficulty finding a husband, and is mocked by her peers. For much of the novel, Doña Maria lives in a state of emotional bondage to her daughter, who consistently rejects her love because she resents her mother's eccentric behavior and refusal to defer to social norms. The oxymoron in this paragraph helps to establish that narrative arc.
It's also important that Wilder is using modern, somewhat skeptical syntax to describe a style of dress that was already antiquated at the time of writing. By describing Doña Maria's attire as a "harness of jewels," he invites the reader to chuckle at modes of dress that now seem strange or even funny. In that sense, this oxymoron is a stylistic reminder that the novel is not attempting to inhabit an 18th-century sensibility or consciousness, but consciously using the historical setting to explore questions of divine will and human agency that are relevant in any time period.
