Preheat the oven to 425°F and line a 9x13-inch sheet pan with parchment paper.
Stretch the dough out to fit the sheet pan. If you're using a ball of pizza dough, let it sit at room temperature first to make it easier to stretch.
Brush the dough with olive oil, especially around the edges.
Spread BBQ sauce evenly over the dough, leaving a small border around the edges.
Sprinkle half of the mozzarella cheese over the sauce.
Add the cooked chicken and sliced red onion.
Top with the remaining mozzarella and Parmesan cheese. Drizzle a little extra BBQ sauce over the top.
Bake for 12–17 minutes, or until the crust is golden and the cheese is melted and bubbly. For extra crisp edges, broil for about 30 seconds at the end, watching closely to prevent burning.
Remove from the oven and finish with an extra drizzle of BBQ sauce and ranch. Garnish with green onion and cilantro, if desired.
Slice and serve warm.