In a small bowl, mix soy sauce, oyster sauce, and cornstarch (if using). Set aside.
Heat olive oil in a large pan over medium-high heat. Add chicken and cook until browned and fully cooked. Remove from pan and set aside.
In the same pan, add sesame oil, garlic, and ginger. Sauté for 30 seconds until fragrant.
Add cabbage, carrots, and bell pepper. Stir-fry for 3-4 minutes until slightly softened.
Return chicken to the pan and pour in the sauce mixture. Stir well to combine.
Cook for another 2 minutes, then remove from heat.
Garnish with chopped green onions and sesame seeds before serving.