Cook the spaghetti according to package instructions in a large pot of salted boiling water. Drain and set aside.
In a skillet or wok, heat the vegetable oil over medium-high heat. Add the thinly sliced chicken breast and stir-fry until cooked through, about 5-7 minutes. Remove the chicken from the skillet and set it aside.
In the same skillet, add the mixed bell peppers and snap peas. Stir-fry for about 3-4 minutes until the vegetables are tender but still crisp.
Add the minced garlic and ginger to the skillet and stir for about 30 seconds until fragrant.
Return the chicken to the skillet and add the cooked spaghetti. Pour in the soy sauce, oyster sauce (if using), sesame oil, sugar, and black pepper. Toss everything together to combine, cooking for an additional 2-3 minutes until everything is heated through.
Serve hot, garnished with chopped green onions and sesame seeds.
For added flavor, consider customizing this dish by adding other vegetables like carrots, broccoli, or mushrooms during the stir-frying process.