In a large skillet over medium heat, cook the ground beef and chopped onion together until the beef is browned and fully cooked.

Drain excess grease if needed.

In a small bowl, mix together the tomato soup, Worcestershire sauce, vinegar, brown sugar, and ketchup.

Pour the sauce mixture into the skillet with the beef and stir to combine.

Simmer for 15-20 minutes.

Add the oatmeal and stir well.

Reduce heat and let simmer for 10–15 minutes, stirring occasionally, until the mixture is thickened and heated through.

Serve hot on buns.