Preheat oven to 400°F. Coat a 2- to 2 1/2-quart baking dish with cooking spray. Remove casings from sausage and crumble with a fork.
Heat oil in a medium skillet over medium heat. Add sausage and onion; cook, stirring, until the onion softens, about 5 minutes. Add soup, spinach and oregano; heat through. Add rice and season with pepper. Transfer to the prepared baking dish; cover with foil. Bake until rice is tender, about 35 minutes. Top with Cheddar and bake uncovered until the cheese is melted, a few minutes longer.