Place the spinach in a large bowl. Cook the oil, shallot, garlic, oregano, salt, pepper, and sugar in a small skillet over medium heat until the shallot is slightly softened, 2 to 3 minutes. Add the lemon juice and swirl to incorporate. Pour the warm dressing over the spinach, add the cheese and olives, and toss gently with tongs to wilt. Serve immediately.