Soak rice noodles in very hot water until pliable, then drain well.
Whisk oyster sauce, soy sauces, fish sauce, brown sugar, lime juice, water, and white pepper in a bowl.
Heat a wok over high heat until smoking, then add the oil.
Sauté garlic and chiles until fragrant, about 30 seconds, stirring constantly.
Add chicken and stir-fry until no longer pink and lightly browned.
Toss in onion and bell pepper; cook until crisp-tender, 2 to 3 minutes.
Add drained noodles and the sauce; stir-fry until glossy and coated.
Stir in tomatoes and sear briefly so they blister without breaking down.
Fold in Thai basil and green onions; toss off heat until basil wilts.
Taste and adjust seasoning with more fish sauce or sugar if needed. Serve hot.