Heat the oil in a wok or large frypan over medium-high heat, and cook the garlic and chillis until golden brown. Mix in chicken and sugar, and season with salt and pepper. Cook until chicken is no longer pink, but not done.
Stir oyster sauce into the wok. Mix in the mushrooms and onions, and continue cooking until onions are tender and chicken juices run clear. Remove from heat and mix in basil. Let sit 2 minutes before serving.