Whisk together the oyster sauce, fish sauce, and sugar in a bowl.
Heat the oil in a wok over medium-high heat until the oil begins to smoke. Add the garlic and serrano peppers, stirring quickly. Stir in the chicken, bell pepper, onion and oyster sauce mixture; cook until the chicken is no longer pink. Raise heat to high and stir in the chilled rice; stir quickly until the sauce is blended with the rice. Use the back of a spoon to break up any rice sticking together.
Remove from heat and mix in the basil leaves. Garnish with sliced cucumber and cilantro as desired.