Heat 2 tablespoons vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and sliced onion and sauté until fragrant and translucent, about 1–2 minutes.
Add the bite-sized chicken pieces, season with salt and pepper, and cook until the chicken is golden and cooked through, about 5–7 minutes, stirring occasionally.
Toss in the sliced red, green, and yellow bell peppers and stir-fry for 3–4 minutes until just tender but still crisp.
In a small bowl, whisk together 2 tablespoons soy sauce, 1 tablespoon oyster sauce, 1 teaspoon cornstarch, and 1/4 cup water until smooth.
Pour the sauce into the skillet, stir to combine, and cook for 2–3 minutes until the sauce thickens and coats the chicken and vegetables. Adjust salt and pepper if needed.
