A simile is a figure of speech in which one thing is compared to another as a means of helping the reader—or, in this case, the audience—understand the thing better through the comparison. Oberon, when speaking of himself as one of the "spirits of another sort," says that he "like a forester, the groves may tread." What this tells us about Oberon is that he, like the forester he refers to, is familiar with and at home in a forest, this word having the connotation, in Shakespeare's time, of a person or creature who inhabits a forest, rather than the modern understanding of a person responsible for a forest's upkeep.
Later, Puck uses a simile to refer to "we fairies" as a whole as "following darkness like a dream." Oberon is included by implication in this simile, although it does not describe him specifically.
There really isn't another simile in this play which describes Oberon. He is referred to by Puck as "jealous Oberon," an epithet. Puck also addresses him by the sobriquets "king of shadows" and "captain of our fairy band," but while these are descriptive phrases which help us to understand Oberon's position within this universe, they are not similes.