In a small bowl, whisk together the oyster sauce, soy sauce, dark soy sauce, fish sauce, and brown sugar until the sugar is dissolved. Set this sauce mixture aside.

Heat the vegetable oil in a large wok or heavy skillet over medium-high heat until shimmering. Add the minced garlic and Thai chilies, stirring constantly until fragrant, about 30 seconds.

Add the thinly sliced shallot and red bell pepper strips to the wok. Stir-fry for 2 minutes until the vegetables just begin to soften and become vibrant.

Increase the heat to high and add the sliced flank steak in a single layer. Let the meat sear undisturbed for 1 minute to develop a brown crust, then stir-fry for another 2 minutes until mostly browned.

Pour the prepared sauce over the beef and vegetables, tossing well to coat everything evenly. Cook for 1 more minute until the sauce bubbles and slightly thickens, glazing the meat.

Remove the wok from the heat and immediately stir in the fresh Thai basil leaves. Toss gently until the basil is just wilted, then serve immediately over hot steamed rice.