In a large skillet, on medium-low heat, sauté mushrooms in olive oil about 6 minutes. Add onion, garlic, and bell pepper, and continue sautéing until onion and pepper are tender, about 4 minutes. Add in sirloin strips, and cook on medium heat for 5–10 minutes, or until no longer pink. Sprinkle with salt, pepper, and oregano. Stir well, cover, and simmer 5 more minutes. Drain mixture.
Stuff pita halves with meat mixture and romaine lettuce, and top with a dollop of yogurt.