Combine ground beef, onion, and garlic in a large stockpot over medium heat. Cook and stir until beef is browned and crumbly, about 10 minutes. Drain and discard excess grease.

Stir tomatoes, tomato sauce, chili powder, salt, and oregano into beef mixture. Break up tomatoes while stirring and bring to a boil, then reduce heat to simmer. Cover and cook, stirring occasionally, for 1 hour.