Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and sliced onion and sauté until fragrant and the onion is translucent, about 2 minutes.
Add the bite-sized chicken pieces, season with salt and pepper, and cook until golden and cooked through, about 5–7 minutes depending on size.
Add 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 the soy sauce, oyster sauce, cornstarch, and 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.
Taste and adjust seasoning with additional salt and pepper if needed, then serve immediately.