Stir together oyster sauce, vinegar, sesame oil, and 2 tablespoons of water in a small bowl. Set aside.
Place chicken in a medium bowl. Season with salt and sprinkle with cornstarch; toss lightly to coat.
Heat 2 tablespoons vegetable oil in a large well-seasoned wok or nonstick skillet over medium-high.
Cook chicken, tossing occasionally, until golden brown and nearly cooked through, 6–8 minutes.
Add red onion, garlic, ginger,gochugaru, and remaining 2 tablespoons of oil.
Cook, tossing, until vegetables are softened, about 2 minutes.
Add corn and cook, tossing often, until tender, about 3 minutes.
Stir in reserved oyster sauce mixture and cook, tossing often, until reduced nearly to a glaze, about 2 minutes.
Taste and season with salt and more gochugaru if needed.
Serve stir-fry with rice, topped with cilantro.