In a large bowl, mix soy sauce, oyster sauce, garlic and pepper. Place Cornish 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 lid from dish and place 1/2 the butter on each hen. Increase oven temperature to 180 C / Gas 4. Basting frequently with the marinade mixture, continue baking 30 minutes or until exterior of the hens are lightly browned and crisp, the interior is no longer pink, and juices run clear.