In a small bowl, whisk together soy sauce, oyster sauce, sugar, and black pepper to create the sauce base.
In a separate small bowl, mix cornstarch and water to create a slurry and set aside.
Heat one tablespoon of oil in a wok or large skillet over high heat until shimmering.
Add chicken cubes in a single layer and sear until golden brown and cooked through, approximately 5-6 minutes; remove chicken from pan and set aside.
Add remaining tablespoon of oil to the pan, then add onions and celery, stir-frying for 2 minutes until slightly softened.
Add minced garlic and ginger to the vegetables and cook for 30 seconds until fragrant.
Return the cooked chicken to the pan and pour in the sauce base, tossing to coat evenly.
Stir the cornstarch slurry once more and pour into the pan, stirring constantly for 1 minute until the sauce thickens and glazes the chicken and vegetables.
Serve immediately.