The Ox Islands (Danish: Okseøer) are two small islands in the Flensburg Fjord, located directly off the Danish town of Sønderhav. The larger of the two islands, the Large Ox Island, is approximately 7.5 hectares in size and was inhabited for a time until 2016. The Small Ox Island has been uninhabited for considerably longer.
Geologically, the islands are the crests of the northern marginal moraine of the former Fjord Glacier – their current island status dates back to the postglacial flooding of the Fjord Valley. The islands were first mentioned in the Danish Royal Register in 1231 under the name "Oxenör minor et major," which indicates their long history of settlement and use.
Over time, the Great Ox Island was used for agricultural and craft purposes. Until the 1970s, members of the Isaack family operated a boatyard there, producing, among other things, clinker dinghies, Folke and Greenland cutters, and sailboats. After 2016, the leased use ended – the shipyard buildings were later removed, and since then the island has been open to the public but uncultivated.
Today, the Great Ox Island boasts a diverse landscape: wooded hilltops, steep cliff slopes, gently sloping northern areas, spit-shaped ridges, and young beach ridges – a charming example of the dynamic coastal formation in the fjord's morphology.