Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the minced garlic and Thai red chilies; sauté for about 30 seconds until fragrant, but be careful not to burn the garlic.
Increase the heat to high and add the sliced beef to the skillet. Stir-fry for about 3-4 minutes until the beef is browned and cooked through.
Add the sliced red bell pepper and cook for an additional 2 minutes until slightly softened.
In a small bowl, mix together the soy sauce, fish sauce, sugar, and oyster sauce. Pour this sauce over the beef and peppers, stirring to coat everything evenly.
Remove the skillet from heat and fold in the fresh Thai basil leaves, allowing them to wilt slightly.
Serve the spicy basil beef hot over cooked jasmine rice, with lime wedges on the side for squeezing.
Notes
Adjust the number of chilies based on your spice preference.