Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add minced garlic and Thai red chilies; stir-fry for about 30 seconds until fragrant—be careful not to burn the garlic.
Increase the heat to high and add the sliced chicken thighs to the skillet. Cook for approximately 5-7 minutes, stirring frequently, until the chicken is browned and cooked through.
Add the sliced onion and bell pepper to the pan. Cook for an additional 2-3 minutes until they start to soften.
In a small bowl, mix together the soy sauce, oyster sauce, fish sauce, and sugar. Pour this sauce over the chicken and vegetables. Stir well to coat everything evenly.
Add the fresh Thai basil leaves and toss everything together just until the basil wilts (about 1 minute).
Remove from heat and serve immediately over cooked jasmine rice.
Notes
Serve in a large bowl over jasmine rice, garnished with extra basil and sliced chilies. Lime juice can be added for freshness.