In a large mixing bowl, combine the flour, sugar, baking powder, and salt.
Gradually add the warm water and vegetable oil, mixing until a dough forms.
Knead the dough on a floured surface for about 5 minutes until smooth.
Cover with a damp cloth and let it rest for 30 minutes.
While the dough is resting, prepare the filling by cooking the ground beef in a skillet over medium heat until browned.
Drain excess fat, then add soy sauce, oyster sauce, hoisin sauce, garlic, ginger, and green onions. Season with salt and pepper.
Stir and cook for another 2-3 minutes until heated through. Remove from heat and let cool.
After the dough has rested, divide it into 8 equal pieces and roll each piece into a circle about 4 inches in diameter.
Place a spoonful of the beef filling in the center of each circle, gather the edges of the dough, and pinch them together to seal the bun.
Place the buns on parchment paper squares and let them rise for 15 minutes.
Set up a steamer over boiling water and steam the buns for 15-20 minutes until fluffy and cooked through.
Serve warm and enjoy your homemade beef bao buns!