
Grease 2-quart casserole dish (8x8-inch pan or 11x7-inch pan).
Cook sausage in a skillet over medium heat, stirring occasionally, until no longer pink; drain.
Spread sausage in prepared baking dish. Top with cheese. Whisk together eggs, milk, Bisquick, dry mustard, and oregano. Pour over sausage.
Cover and refrigerate at least 4 hours but no longer than 24 hours.
Preheat oven to 350°F. Bake uncovered 50 to 60 minutes, or until knife inserted in center comes out clean.