Place rice noodles in a large bowl and cover in boiling water. Soak until not quite cooked, then remove from water and rinse under cold water to prevent over cooking.
Combine oyster sauce, both soy sauces, sugar, and 1 Tbsp. water in a small bowl, whisking to combine. Set aside.
In a large pan, briefly fry garlic and chiles in oil until very fragrant.
Add onion and briefly fry.
Add chicken and fish sauce. Fry until chicken is just cooked through.
Add green onion, noodles, and sauce. Cook briefly, tossing to coat noodles and reduce sauce.
Remove from heat, add basil, and toss to combine and wilt basil. Serve immediately.
Made a double batch with tofu instead of chicken; no chiles or green onions; and only regular soy sauce. Added an additional 2 Tbsp. sugar and 2 Tbsp. rice wine vinegar because it was incredibly salty. The sugar and vinegar made it better, but this is still not that great.
Made using 1/4 red bell pepper instead of chiles and tofu instead of chicken. Used Genovese basil and only regular soy sauce. It was fine, but not amazing. I think even more basil, or a more potent variety of basil, would have made this much better. The sauce is also underwhelming, salt being the most prominent flavor.
Made a double batch with young Vidalia onions, a piece of a Jalapeno, chicken thighs, and closer to 3 cups of a mix of Genovese and lemon basil. Only used generic 'soy sauce', plus an extra teaspoon (for a total of 5) sugar to compensate for the lack of dark soy sauce. Overall much better than previous makes. After adding a bit more soy sauce, I would even hazard to call it 'very good', but not quite perfect. Will definitely continue to make this, but never again with just tofu.