In a large bowl, mix soy sauce, oyster sauce, garlic, and pepper. Place Cornish game hens in the mixture, and marinate 10 minutes.
Transfer hens to a medium baking dish, and cover with the marinade mixture. Cover dish, and bake 30 minutes in the preheated oven.
Remove cover from dish, and place 1/2 the butter on each hen. Increase oven temperature to 350 degrees F (175 degrees C). Basting frequently with the marinade mixture, continue baking 30 minutes, or until exterior of the hens is lightly browned and crisp, the interior is no longer pink, and juices run clear.