Start by mincing your garlic finely and slicing the Thai chilies thinly—this releases their aroma and heat. Shell and devein the shrimp, then set everything aside on your prep station.
Heat your wok or large skillet over high heat until just smoking. Add the oil and swirl to coat the surface evenly.
Toss in the minced garlic and sliced chilies immediately. Sauté for about 15 seconds until fragrant, hearing a lively crackle and smelling the bright, pungent aroma.
Spread the shrimp in a single layer in the pan. Let them sear undisturbed for 1-2 minutes until they start turning pink and develop a slight caramelization on the edges.
Stir the shrimp around and cook for another 2 minutes until they are fully pink and firm, with a smoky aroma wafting up from the pan.
Pour in the fish sauce, add the sugar, and include the oyster sauce if using. Toss everything together quickly for about 30 seconds until the sauce becomes glossy and fragrant, coating the shrimp evenly.
Remove the pan from heat immediately and toss in the chopped basil leaves. Stir for about 10 seconds until the basil wilts, turns bright green, and releases its peppery aroma.
Transfer the stir-fry to a warm plate and let it rest for a minute to allow flavors to settle. Serve immediately, garnished with extra basil if desired.