Heat olive oil in a casserole over medium-high heat until smoking. Brown the oxtail in batches and set aside. Add onion, garlic and green pepper to the pot; cook, stirring constantly, until the onion has softened and turned translucent, about 5 minutes.
Pour in white wine, bring to a simmer and cook for 5 minutes. Add browned oxtail, beef stock, chocolate and bay leaves. Return to a simmer, then reduce heat to medium-low, cover and simmer for 3 hours.
Season with salt and paprika, stir in carrots and allow to simmer until carrots are tender.