Heat wok or pan to high. Once the wok gets really hot (smoke is coming out), add 1 tablespoon oil. Fry marinated chicken for one minute.
Take chicken out of pan once you see some browning. Your chicken should be 80% done (don't worry we will stir fry it again). Place on a separate plate.
Heat wok on high again and add another tablespoon of oil. Fry garlic, onions, red bell pepper, and Thai chilis.
Add chicken (from step 5) back in. Then, add lemongrass (optional), oyster sauce, soy sauce, water, and sugar to the stir fry and mix together quickly so the ingredients don't burn.
Fry for 2-3 minutes until the chicken is cooked through and add in the thai basil. Hope you enjoy with hot white rice or a fresh salad!