Coat a large nonstick skillet with cooking spray; add oil. Place over medium-high heat until hot. Add onion and next 4 ingredients; saute until crisp-tender. Add oregano and next 3 ingredients; saute 1 minute.
Add chicken broth and next 6 ingredients, stirring well. Bring to a boil; cover, reduce heat, and simmer 20 minutes. Add cilantro; cover and cook 5 additional minutes. To serve, ladle into bowls; top evenly with sour cream, cheese, and green onions.