Heat the vegetable oil in a large skillet or wok over high heat until shimmering.
Add the minced garlic and sliced bird chilies to the pan, stirring quickly for about 15 seconds, or until fragrant.
Increase the heat, and add the sliced beef to the pan. Stir-fry for 2-3 minutes until the beef is browned but not overcooked.
Add the sliced red bell pepper, soy sauce, oyster sauce, sugar, and black pepper. Continue to stir-fry for another 2 minutes, ensuring everything is well-coated and heated through.
Remove the stir-fry from heat, and gently fold in the Thai basil leaves until just wilted.
Serve the stir-fry hot over cooked jasmine rice, garnished with additional fresh basil leaves if desired.
Notes
Adjust the number of chilies based on your heat preference.