Slice the chicken breast into thin strips, 1/4-inch (5 mm) thick by 2 inches (5 cm) long and about 1 inch (2.5 cm) wide. To achieve very thin slices, put the chicken breasts into the freezer for about 20 minutes to harden slightly, then slice. Set aside.
Pour oil into the cold wok and add the garlic and chiles. Cook over high heat until the aromas start to release. Add the sliced chicken and stir-fry for about 2 minutes.
Add the fish sauce, sugar, and soy sauce and stir-fry for another minute. Add the oyster sauce and 2 tablespoons of water and cook for 30 seconds.
Add the peppers and 16 of the basil leaves and stir-fry for 2 minutes, until the peppers start to soften.
Dissolve the cornstarch in 1 tablespoon of cold water, add to the wok, and stir-fry for 1 minute, until the sauce thickens.
Remove from heat and transfer to a serving dish. Garnish with the remaining basil leaves and serve immediately with steamed jasmine rice.