Cook the ramen noodles according to package instructions (without seasoning packets). Drain and set aside.
In a large skillet or wok, heat the vegetable oil over medium-high heat.
Add the chicken pieces and season with salt and pepper. Cook until browned and cooked through, about 5-6 minutes.
Push the chicken to the side of the pan. Add bell pepper, carrots, and broccoli. Sauté for 3-4 minutes until tender-crisp.
Add garlic and cook for another 30 seconds until fragrant.
In a bowl, whisk together the soy sauce, oyster sauce, brown sugar, rice vinegar, and sesame oil.
Pour the sauce into the skillet and toss to coat the chicken and veggies. If using, stir in the cornstarch slurry to thicken the sauce slightly.
Add the cooked ramen noodles and toss everything together until well combined and heated through.
Garnish with sesame seeds and green onions before serving, if desired.