In a large sauté pan, heat the oil over medium-high heat. Add the onion and cook, stirring often, until softened, about 3 minutes. Add the beef and cook, stirring often to break up the meat, until no longer pink, about 6 minutes. Stir in the hash browns, Cheddar, 2 teaspoons salt, and the pepper. Stir until well combined. Taste and adjust the seasoning. Set aside.