Stir peanut butter, vegetable oil, vinegar, soy sauce, garlic, sugar, and cayenne pepper together in a bowl until smooth.
Bring a pot of water to a boil. Cook ramen noodles in boiling water until tender yet firm to the bit, about 3 minutes; drain and transfer noodles to a bowl. Pour peanut butter sauce over the noodles and toss to coat. Stir cucumber and green onion into the noodle mixture; garnish with cilantro and peanuts.
