Cook the Shanghai noodles according to package instructions. Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat.
Add the minced garlic and sliced onion to the skillet, sautéing for about 1 minute until fragrant.
Add the shredded cabbage, bean sprouts, carrot, and bell pepper to the skillet. Stir-fry for 3-4 minutes until the vegetables are tender-crisp.
Push the vegetables to one side of the skillet. Add the cooked noodles to the other side and pour the soy sauce, oyster sauce, and sesame oil over the noodles.
Toss everything together, mixing the noodles and vegetables thoroughly. Season with salt and pepper to taste.
Stir in the chopped green onions and cook for an additional 1-2 minutes until everything is heated through.
Serve hot and enjoy your Shanghai fried noodles!