Rinse the jasmine rice until the water runs clear, then cook it with water and a pinch of salt according to package instructions. Keep warm.
Slice the beef thinly against the grain and set aside.
In a small bowl, mix oyster sauce, soy sauce, fish sauce, sugar, and cornstarch slurry.
Heat oil in a wok or large skillet over high heat.
Add garlic and chilies and stir-fry for a few seconds until fragrant.
Add the beef and cook quickly, stirring constantly, until just browned.
Add onion and bell pepper and stir-fry for 1–2 minutes until slightly tender.
Pour in the sauce and cook until it thickens and coats the beef evenly.
Turn off the heat and stir in the Thai basil until just wilted.
Serve immediately over jasmine rice.