Heat oil in a skillet over medium heat. Cook and stir green onion, garlic, and ginger in hot oil until fragrant, about 1 minute. Crumble ground pork into skillet, breaking it into small pieces with the back of a wooden spoon as you stir. Pour soy sauce over the pork mixture; cook and stir until the pork is completely browned, 5 to 7 minutes. Stir peas into the pork mixture; cook and stir until the peas are softened, 3 to 5 minutes. Fold tofu into the pork mixture; cook until the tofu is hot, about 3 minutes.
Whisk water, oyster sauce, cornstarch, salt, and white pepper together in a bowl; stir gently into the pork and tofu mixture. Cook until the sauce thickens, about 5 minutes.