Heat the olive oil in a large skillet over medium heat. Add the onion and red bell pepper and cook until softened, about 5 minutes. Add the garlic and cook for an additional minute. Stir in the smoked paprika and oregano.
Add the rice to the skillet and stir to coat with the oil mixture. Pour in the seafood stock, tomato paste, bay leaf, saffron threads (if using), salt and pepper. Bring to a boil then reduce heat to low.
Cover skillet with a tight fitting lid and cook for 15 minutes without stirring.
Uncover skillet and add mussels (if using). Cover again with lid and continue cooking until all liquid is absorbed and mussels are cooked through, about 10 minutes more.
Remove from heat and let stand 5 minutes before serving.