Combine oat cereal, corn flake cereal, and peanuts together in a large bowl.
Mix brown sugar, corn syrup, and butter together in a saucepan; bring to a boil. Remove from heat and stir in peanut butter and vanilla extract until smooth. Pour peanut butter mixture over cereal mixture; stir to coat. Press coated cereal mixture into a 9x13-inch casserole dish. Refrigerate until set, at least 1 hour. Cut into squares.
