In a small bowl, mix together the soy sauce, oyster sauce, fish sauce, and brown sugar. Set aside.
In a large wok or frying pan, heat the vegetable oil over medium-high heat.
Add the sliced beef to the hot oil in a single layer. Let it sear for about 1-2 minutes without stirring to develop a nice brown color, then stir-fry for another 1-2 minutes until nearly cooked through.
Toss in the minced garlic and sliced red chilies, and stir-fry for an additional 30 seconds until fragrant.
Add the sliced bell pepper and stir-fry for another minute until slightly softened.
Pour the prepared sauce over the beef and vegetables, then add the fresh Thai basil leaves. Stir everything together and cook for another 1-2 minutes until the sauce is heated through and the basil leaves are wilted.
Remove from heat and serve immediately over cooked jasmine rice.
Notes
Adjust the number of chilies based on your spice preference.