Cook ramen noodles according to package instructions, drain, and set aside.
Heat vegetable oil in a large skillet or wok over medium-high heat.
Add garlic and ginger, sauté for 30 seconds until fragrant.
Add the cooked chicken and stir-fry for 2–3 minutes.
Add all the vegetables and cook for 4–5 minutes, stirring constantly, until they are tender-crisp.
Add the cooked ramen noodles to the skillet.
Pour in soy sauce, oyster sauce (if using), and sesame oil. Toss everything together until evenly coated.
Season with black pepper to taste.
Remove from heat and garnish with sliced green onions and sesame seeds.