In a large straight-sided skillet or a wide, shallow pot, heat 2 tablespoons oil over medium-high. Add onion and cook until translucent, about 3 minutes. Add beans, tomatoes, broth, and oregano, stirring to combine; season with salt and pepper. Bring to a boil. Reduce heat to medium, cover, and simmer, stirring occasionally, until beans are very tender, 18 to 20 minutes. Stir in remaining 3 tablespoons oil and season with salt and pepper; serve.