Oscar the Grouch is a (former) main antagonist of Sesame Street franchise.
He always lives up to the species he is, a grouch, who is miserable, argumentative, unhygienic, rude, and just plain grouchy. He is the main antagonist from 1969 to 1990, one of the supporting characters from 1991 to 2005, one of the main protagonists from 2006 to present.
He is voiced by Caroll Spinney from 1969 to 2018 and currently voiced by Eric Jacobson, who also voices Grover, Fozzie Bear, and Miss Piggy.