Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and chopped Thai chilies to the skillet, sauté for about 30 seconds until fragrant.
Increase the heat to high and add the ground beef, stirring well to break it apart. Cook until browned and any liquid has evaporated, about 5-7 minutes.
Stir in the sliced onions and bell peppers, cooking for another 2-3 minutes until they start to soften.
In a small bowl, mix together the soy sauce, fish sauce, oyster sauce, and sugar. Pour this sauce over the beef mixture, stirring well to coat.
Add the fresh Thai basil leaves and toss everything together until the basil wilts, about 1 minute.
Remove from heat and serve immediately over a bed of jasmine rice.
Notes
Adjust the number of chilies based on your spice preference.