Prep the shrimp: Pat shrimp dry with paper towels.
This helps them sear instead of steam. If you like, toss them with a pinch of salt and pepper.
Make the sauce: In a small bowl, stir together oyster sauce, soy sauce, fish sauce, brown sugar, water, and rice vinegar or lime juice. Taste and adjust sweetness or saltiness.
If you want a slightly thicker sauce, dissolve 1 teaspoon cornstarch into the sauce now.
Heat the wok or skillet: Set a large wok or skillet over medium-high to high heat. Add oil and swirl to coat. You want the pan hot so the shrimp and veggies cook fast and stay crisp.
Sauté aromatics: Add garlic and chilies.
Stir-fry for 20–30 seconds until fragrant. Keep them moving so they don’t burn.
Cook the shrimp: Add the shrimp in a single layer. Sear for 1–2 minutes until the underside turns pink and slightly golden.
Flip and cook another 30–60 seconds. The shrimp should be just shy of done. Transfer to a plate to avoid overcooking.
Stir-fry the veggies: Add onion and bell pepper (and any firm veggies like green beans).
Stir-fry 2–3 minutes until crisp-tender. If using softer veggies like zucchini or snow peas, add them during the last minute.
Combine and sauce: Return shrimp to the pan. Pour in the sauce.
Toss everything together for 30–60 seconds until the shrimp finish cooking and the sauce turns glossy.
Add the basil: Turn off the heat. Toss in the Thai basil leaves and stir until just wilted. The residual heat is enough to release that signature aroma.
Taste and finish: Adjust with a splash more fish sauce for salt, a pinch of sugar for balance, or a squeeze of lime for brightness.
Serve immediately over hot jasmine rice or with noodles.