Cook the lo mein noodles according to the package directions. Drain and rinse with cold water. Set aside.
In a small bowl, whisk together soy sauce, oyster sauce, brown sugar, sesame oil, and cornstarch.
Heat vegetable oil in a large skillet or wok over medium-high heat. Add chicken and cook until browned and cooked through, about 5-7 minutes. Remove chicken and set aside.
In the same skillet, add garlic and onion. Cook until onion softens, about 2 minutes. Add carrots, bell peppers, mushrooms, and cabbage. Stir-fry until vegetables are tender-crisp, about 3-5 minutes.
Add cooked chicken back to the skillet with vegetables. Pour sauce over and stir to coat.
Add cooked lo mein noodles to the skillet. Toss to coat noodles with sauce and heat through, about 2-3 minutes.
Garnish with sliced green onions and sesame seeds, if desired. Serve immediately.