Preheat the oven to 500˚. Line a rimmed baking sheet with foil. Trim the ends from the bread, then cut into 4 equal pieces; split each piece in half. Arrange the bread cut-sides up on the prepared baking sheet.
Mix 3 tablespoons olive oil with the garlic and oregano. Brush the oil on the cut sides of the bread, then season with salt and pepper. Bake until lightly toasted, 4 to 5 minutes. Let cool slightly.
Heat the remaining 1 tablespoon olive oil in a large skillet over medium-high heat. Add the sausage and cook, stirring, until crisp, about 5 minutes. Transfer to a plate with a slotted spoon. Add the onion and bell pepper to the skillet and season with salt and pepper. Cook, stirring occasionally, until tender and lightly browned, about 5 minutes. Stir in 1/4 cup barbecue sauce and cook until the vegetables are glazed, about 2 minutes.
Combine the Monterey Jack and cheddar in a medium bowl. Sprinkle about 1/4 cup of the cheese mixture on each bread slice, then top with the onion-pepper mixture, sausage and remaining cheese. Bake until the cheese is melted and bubbling, about 8 minutes.
Microwave the remaining 1/4 cup barbecue sauce in a microwave-safe bowl until warm, about 30 seconds. Drizzle on the pizzas and sprinkle with cilantro.