Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the chicken pieces and stir-fry for about 5-7 minutes, or until the chicken is cooked through and lightly browned.
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, cooking for an additional 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 coat everything evenly. Cook for another 2 minutes to heat through.
Remove the skillet from heat and stir in the fresh Thai basil leaves until wilted.
Serve the Thai basil chicken over cooked jasmine rice.