Oak Island , North Carolina is a town located mostly on Oak Island ( which also contains the town of Caswell Beach and the North Carolina Baptist Assembly at Fort Caswell ) in Brunswick County , North Carolina ; a small part of the town extends onto the mainland north of the island 's bridge . Oak Island , North Carolina is a town located mostly on Oak Island ( which also contains the town of Caswell Beach and the North Carolina Baptist Assembly at Fort Caswell ) in Brunswick County , North Carolina , United States ; a small part of the town extends onto the mainland north of the island 's bridge .