Bring a large pot of lightly salted water to a boil; cook rotini pasta at a boil until tender yet firm to the bite, about 8 minutes; drain and rinse with cold water to cool.

Whisk sugar, salt, and hot water in a large salad bowl until sugar has dissolved. Stir vinegar, vegetable oil, and oregano into sugar mixture to make a dressing. Mix tomato, red bell pepper, and onion in the dressing. Gently fold rotini into dressing and vegetables, mix in Cheddar cheese, and season with black pepper.