Combine oyster sauce, soy sauce, sesame oil, sugar, and water. Set aside.
Char the Bok Choy
Heat a wok or wide skillet over the highest heat possible until smoking. Add oil, then place bok choy cut-side down in a single layer. Sear undisturbed 2 minutes until the cut face is deeply charred and caramelized. Flip and cook 1 more minute.
Add the Sauce
Push bok choy to the sides. Add minced garlic to the center and cook 20 seconds.
Pour sauce over everything and toss to coat. Let it bubble and reduce 30 seconds until the bok choy is glossy and the sauce clings.
Plate and Finish
Arrange bok choy cut-side up so the char marks show. Spoon remaining sauce over the top. Finish with toasted sesame seeds and an optional chili oil drizzle.