Crumble the ground beef into a skillet over medium heat; add onion, salt, and pepper. Cook and stir the ground beef until no longer pink, 7 to 10 minutes.
Stir oats into beef mixture to coat in beef drippings.
Pour beef stock over beef mixture; bring to a boil, place cover on the skillet, and reduce heat to low. Cook at a simmer until the oats are cooked, about 30 minutes.
Stir Worcestershire sauce through the beef mixture to serve.