Heat the olive oil in a large skillet over medium-high heat. Add the garlic and half of the scallions and cook, stirring, 1 minute. Add the tomatoes and oregano and cook, stirring, 3 minutes. Stir in 21/2 cups water, the orzo, 1/2 teaspoon salt, and pepper to taste. Bring to a boil, then reduce the heat to medium low and stir in the beans. Cover and simmer until most of the liquid is absorbed and the orzo is tender, about 10 minutes.
Add the bell pepper and continue cooking, covered, until tender, about 3 minutes. Stir in the tuna, lemon juice and parsley. Season with salt and pepper. Top with the remaining scallions.