Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the sliced chicken and cook for about 5-7 minutes, or until browned and cooked through.
Add the minced garlic and sliced chilies to the skillet, stirring for about 30 seconds until fragrant.
Add the sliced red bell pepper and green beans to the skillet. Stir-fry for another 3-4 minutes until the vegetables are tender-crisp.
In a small bowl, mix together the soy sauce, oyster sauce, fish sauce, and sugar. Pour this sauce over the chicken and vegetables, stirring to combine. Cook for an additional 2 minutes to heat through.
Remove the skillet from heat and stir in the fresh Thai basil leaves until just wilted.
Serve the stir-fry over cooked jasmine rice.