Rinse the rice with water to remove excess starch, set aside.
Heat a large deep pan over medium heat. Add two tablespoons olive oil, onions, peppers and mushrooms and cook until the vegetables soften and the onions become lightly golden, about 5 minutes.
Add the garlic and cook until it becomes fragrant, about 1 more minute.
Transfer the rinsed rice on top of the vegetables. Season with oregano, ½ teaspoon salt and ½ teaspoon pepper and cook with the vegetables for 2-3 minutes, stirring frequently. Add the chicken broth on top, bring to a boil, then simmer and cook for 15 minutes covered with a tight-fitting lid. Remove rice from heat and keep covered for 5 minutes before adding the shrimp.
Combine the shrimp with the remaining olive oil, salt and pepper in a small bowl. Nestle the shrimp into the rice when done cooking, cover and allow it to steam with the rice until pink and opaque, about 5-7 minutes.
Fluff the rice with the shrimp and garnish with cilantro, if desired.