In a small bowl, combine fish sauce, oyster sauce, sugar and water. Mix well and set aside.
Heat 1 tbsp oil in a wok (or skillet) over med-high heat and add garlic and chilies. Stir-fry for 10 seconds, then add the beef. Stir-fry until the beef is no longer in pink, about 2 minutes. Transfer the beef to a plate and set aside.
Add the remaining 1 tablespoon oil to the wok. Add the onion and the sauce. Stir-fry for 1-2 minutes until the onions are just soft.
Add the beef and heat through. Add the basil leaves and remove the wok from the heat and shake everything to mix well. Serve immediately with jasmin rice.
If you want this less spicy, use finger-long red chilies instead of Thai bird's eye chilies