Position racks in upper and lower thirds of oven; preheat to 425°F.

Place squash, pancetta and garlic on a rimmed baking sheet. Toss with 1 tablespoon oil, oregano and ground pepper to coat well. Roast on the upper rack, stirring once, until the squash is tender and the pancetta is golden brown and crisp, about 30 minutes. Add vinegar and crushed red pepper and toss to combine.

Meanwhile, arrange baguette slices on another baking sheet and brush with the remaining 2 tablespoons oil. Bake on the lower rack until golden brown, about 15 minutes.

Mix ricotta, 3 tablespoons chives and 2 tablespoons Parmesan in a small bowl. Spread on the crostini. Top with the squash mixture, pistachios, the remaining 1 tablespoon chives and 2 tablespoons Parmesan. Serve warm.