In a small bowl, mix soy sauce, oyster sauce, sugar, and water. Set aside.
Heat oil in a large skillet or wok over high heat.
Add garlic and chilies and stir-fry for about 10–15 seconds until fragrant.
Add sliced chicken and cook, stirring frequently, until fully cooked and lightly browned.
Pour in the prepared sauce and toss to coat the chicken evenly. Cook for 1–2 minutes until the sauce lightly coats the chicken.
Add Thai basil leaves and stir just until wilted.
Remove from heat and serve immediately.