Osea Island, formerly also Osey, is an inhabited island in the estuary of the River Blackwater, Essex, East England. It is approximately 380 acres (1.5 km2) in size and is connected to the north bank of the river by a causeway that is covered at high water. The island's population is included in the civil parish of Heybridge.

The Blackwater Estuary is the estuary of the River Blackwater between Maldon and West Mersea in Essex. It is a 5,538 hectare biological Site of Special Scientific Interest (SSSI). An area of 4,395 hectares is also designated a Ramsar Wetland of International Importance, and a Special Protection Area 1,099 hectares is a National Nature Reserve. Tollesbury Wick and part of Abbotts Hall Farm, both nature reserve managed by the Essex Wildlife Trust, are in the SSSI.