Thinly slice the beef against the grain and set aside. Mince the garlic, chop the chilies, and wash the Thai basil leaves.
In a small bowl, mix oyster sauce, soy sauce, fish sauce, and brown sugar. Stir well to combine.
Heat a wok or skillet over high heat. Add vegetable oil and swirl to coat the surface.
Add the beef and sear for 1-2 minutes until browned but still tender. Remove from the pan and set aside.
In the same pan, add garlic and chilies. Stir-fry for 30 seconds until fragrant.
Return the beef to the pan and pour in the sauce mixture. Toss everything together for 1-2 minutes, allowing the beef to absorb the flavors.
Turn off the heat and add the Thai basil leaves. Stir until just wilted.
Serve immediately over steamed jasmine rice. Top with a fried egg if desired.