In a large bowl, combine the chicken pieces with soy sauce, oyster sauce, black pepper, and sugar. Mix well and let marinate for at least 30 minutes.
Heat the vegetable oil in a large skillet or wok over medium-high heat. Add the minced garlic and ginger, stirring for about 30 seconds until fragrant.
Add the marinated chicken to the skillet, cooking for about 5-7 minutes until the chicken is browned and cooked through.
Add the sliced onion and bell pepper to the skillet, stirring frequently for another 3-4 minutes until the vegetables are tender but still crisp.
Taste and adjust seasoning if necessary, adding more black pepper if desired.
Remove from heat and garnish with chopped green onions. Serve hot over cooked rice.