Position a rack in the middle of the oven; preheat to 400°. Cut out the center of the bread, leaving a 1-inch border. Using your fingers, hollow out the bread to leave about 1 inch at the bottom.
Place the cut-out bread on a work surface; cut into rectangular pieces to use as dippers. Place on a large parchment-lined baking sheet. Drizzle with 2 tbsp. EVOO; toss. Place the bread bowl on the baking sheet. Bake until the inside of the bowl is toasted, 5 minutes.
Transfer the bread bowl to a platter; rub the cut garlic on the inside. Toss the dippers again and bake for 5 minutes more. Transfer to the platter; increase oven heat to 450°.
Heat a dutch oven over medium heat; add the remaining 1 tbsp. EVOO. Add the onion and cook, stirring, for 5 minutes. Chop the remaining garlic clove; add to the pot. Add the sausage; cook, breaking it into chunks, until beginning to brown, about 5 minutes. Add the pizza sauce. Cook, stirring, until heated through, 5 minutes.
Return the bread bowl to the baking sheet; ladle in the sauce. Top with the 2 cheeses. Bake until the cheese browns, about 10 minutes. Top with the oregano.
Nestle the bread bowl among the dippers on the platter.