Preheat the oven to 400 degrees F (200 degrees C). Lightly grease a 9x13-inch baking dish.
Season chicken thighs all over with salt and pepper; place into the prepared baking dish with the skin facing up.
Bake in the preheated oven for 25 minutes.
Meanwhile, mix oyster sauce, sugar, water, and garlic together in a bowl. Taste and add more sugar if not sweet enough.
Pour sauce over chicken and continue to bake until no longer pink at the bone and the juices run clear, about 20 more minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).
