Heat oil in a wok or a large skillet over high heat until a drop of water sizzles. Add chili, garlic, and onion. Quickly sauté until fragrant, about 30 seconds.
Add snow peas, mushrooms, and bell pepper. Sauté for about 2 minutes.
Switch to medium-high heat. Stir in beef; separate the slices using a spatula. Whisk oyster sauce, fish sauce, and brown sugar in a small bowl and add the sauce in the wok. Cook for 2 more minutes or stir fry beef until tender, stirring frequently.
Turn off the heat and toss basil leaves in the beef stir fry. Serve over rice.