What literary devices does Aphra Behn use in Oroonoko?

Part of the reason why Oroonoko remains of interest to modern readers is because of its ambiguous stance on slavery. The very nature of its title, Oroonoko, or The Royal Slave, is an active point of contention that best describes its sense of conflict. In particular, imagery and third-person narration work to create various, dynamic arguments for where Behn herself and her text stand with respect to the institution.
When we are first introduced to Oroonoko, he is described as being

not of that brown rusty black which most of that nation are, but of perfect ebony, or polished jett. ... His nose was rising and Roman, instead of African and flat. His mouth the finest shaped that could be seen; far from those great turn'd lips, which are so natural to the rest of the negroes. The whole proportion and air of his face was so nobly and exactly form'd, that bating his colour, there could be nothing in nature more beautiful, agreeable and handsome ...

By emphasizing his Eurocentric features, the narrator puts obvious effort into setting Oroonoko apart from other black characters. Similarly, his intelligence and morals are praised for their European sensibilities. Most of Oroonoko's nobility is attributed to his non-Africanness throughout the novel, especially once he is captured, which in and of itself complicates the discussion.
Likewise, the narrator remains uncritical of slavery throughout the narrative. From the description of the colonial slave trade in the opening exposition, to Oroonoko's detainment of prisoners of war, to his very own enslavement, there is no definitive evidence to show how the narrator feels about the subject, apart from the fact that they would rather avoid direct confrontation with it:

Some have commended this act ... but I will spare my sense of it, and leave it to my reader to judge as he pleases.

Oroonoko, written by Aphra Behn, is about a prince who was taken from Africa and forced from his high position into ultimate powerlessness; he was enslaved, moving from a position of highest esteem to a position of lowest esteem in a moment. Behn, a white female author, attempts to share his story in a realistic manner, though she often romanticizes his life through her lavish descriptions. One of these descriptions is seen as she uses imagery to present Oroonoko's physical appearance. She writes,

His face was not of that brown rusty black which most of that nation are, but of perfect ebony, or polished jett. His eyes were the most awful that cou'd be seen, and very piercing; the white of 'em being like snow, as were his teeth. His nose was rising and Roman, instead of African and flat. His mouth the finest shaped that could be seen; far from those great turn'd lips, which are so natural to the rest of the negroes. The whole proportion and air of his face was so nobly and exactly form'd, that bating his colour, there could be nothing in nature more beautiful, agreeable and handsome. There was no one grace wanting, that bears the standard of true beauty. His hair came down to his shoulders, by the aids of art, which was by pulling it out with a quill, and keeping it comb'd; of which he took particular care.

Upon meeting Oroonoko, she describes his appearance with vivid diction choices, employing words such as "polished jett," "nobly and exactly form'd," and "no grace wanting," to show Oroonoko's majestic appearance. In this passage, we also see Behn employing a simile, describing the white of his eyes (and his teeth) as being "white . . . like snow." Additionally, she employs hyperbole, or extreme exaggeration, in describing him as "so nobly and exactly form'd . . . there could be nothing in nature more beautiful, agreeable and handsome." These similes and hyperboles offer readers more clarity about Oroonoko's appearance and heighten his grandeur. Her description makes him out to be so perfect in appearance that he seems almost holy, despite her admission that his culture behaved differently than her Christian culture. For instance, she writes of Imoinda, the beauty of Oroonoko's homeland, who fell in love with Prince Oroonoko but was loved by the king and forced to become his concubine. Through this we learn of Oroonoko's culture's practices of marrying multiple women, something Behn would have opposed. Even though Oroonoko's culture follows "pagan" practices, the narrator speaks highly of the culture, using a positive tone to describe Oroonoko and his life accomplishments, despite his immense struggles.
Another literary device seen in the text is Behn's use of allusions. For instance, she alludes to Imoinda, Oroonoko's great love, as "black Venus." Oroonoko is called "black Mars." These allusions to Roman gods once more magnify Oroonoko's power and majesty. This is seen in the following passage introducing Imoinda:

This old dead hero had one only daughter left of his race, a beauty, that to describe her truly, one need say only, she was female to the noble male; the beautiful black Venus to our young Mars; as charming in her person as he, and of delicate virtues. I have seen a hundred white men sighing after her, and making a thousand vows at her feet, all in vain, and unsuccessful. And she was indeed too great for any but a prince of her own nation to adore.

Not only does this passage contain allusions, but it once more includes hyperbole, in describing the "hundred white men sighing after her, and making a thousand vows at her feet, all in vain." This is a clear exaggeration made for dramatic effect to emphasize her point that Imoinda was a great beauty.