Pre-heat the oven to 375 degrees. In a large skillet over medium-high heat, add the ground beef and onion. Cook until the ground beef has browned. Drain any excess liquid and set aside.
Lightly grease a casserole dish. Add soup, milk, and oregano. Add the ground beef mixture and mix well. Top with tater tots. Sprinkle cheese over the top of the tater tots.
Cover and place into the oven. Bake covered for 40 minutes. After 40 minutes uncover and bake for an additional 15 minutes or until bubbly and the cheese browned.