Cook the noodles according to package instructions until al dente. Drain and rinse under cold water to stop the cooking process. Set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the minced garlic and sliced onion, and sauté for about 2 minutes until fragrant.
Add the bell pepper and snap peas to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender but still crisp.
Push the vegetables to one side of the skillet. If using, add the cooked protein to the other side and heat through for about 2 minutes.
Add the cooked noodles to the skillet, followed by the soy sauce, oyster sauce (if using), and sesame oil. Toss everything together until the noodles are evenly coated and heated through, about 2-3 minutes.
Season with salt and pepper to taste. Stir in the chopped green onions just before serving.