When we think of oxymorons, we often think of two-word phrases with seemingly opposite components, like "alone together" or "sweet sorrow." But in some instances, a single word is all it takes.
Let’s get this one out of the way. When we learn about oxymorons, bittersweet is often one of the first words given as an example. The word combines two of the five taste sensations, the others being salty, sour, and savory (umami).
Whether bitter is really the opposite of sweet on the taste spectrum is arguable, but they are often contrasted since bitter is associated with medicines or some vegetables (like radishes) while sweet is associated with, well, sweets (candy and desserts).
Bittersweet is the word for a pleasant feeling that is nonetheless marked by sadness or regret, such as the feeling one might have upon graduating from school, where many friendships were made. In cuisine, it describes things like dark chocolate, which is thought of as sweet but in reality contains little sugar, so has a sharper taste than regular chocolate.