In a wok, heat 2 tablespoons of oil over medium heat. Once oil is hot, fry the pork, in batches, 5 to 8 minutes or until pork is cooked through. Remove pork from wok, and set aside. Add 1 tablespoon to hot pan, add garlic. Sauté garlic 1 to 2 minutes, or until fragrant. Add vegetables, except leeks, and stir fry 7 to 8 minutes, until vegetables are tender-crisp. Add leeks and stir fry 1 to 2 minutes more. Add water, and bring to a boil. Add oyster sauce, and stir to combine. Allow to reduce 3 to 4 minutes, until reaches a sauce consistency. Return pork to the pan and stir to combine. Serve with hot steamed rice.