Juliet uses the following oxymorons to describe Romeo, "Beautiful tyrant! Fiend angelical!/Dove-feathered raven, wolvish-ravening lamb!"
The reason behind these oxymorons is to show the confusion and conflict Juliet has in regards to her feelings for Romeo. Half of her loves her new husband, but the other half hates him for killing her cousin. She is struggling with what to do with her feelings. If she sides with Romeo, she feels like a traitor to her cousin. If she sides with Tybalt, she feels like a traitor to her husband. The use of contrasting phrases emphasizes her torn feelings.