Prepare all ingredients, chopping vegetables and measuring sauce components.

Slice onions and mushrooms, and mince garlic for the stir-fry.

Boil water and cook udon noodles for 2 minutes until slightly softened.

Mix soy sauce, sugar, water, vinegar, chili oil, and oyster sauce in a bowl.

Heat chili oil in a pan and sauté minced garlic until fragrant.

Add chopped vegetables to the pan and stir-fry until softened.

Pour the prepared sauce into the pan and simmer until slightly thickened.

Drain the cooked udon noodles and add them to the pan.

Heat chili oil in another pan and stir-fry pork with pepper until cooked.

Combine the pork with the noodles and vegetables, stir-frying briefly.