Old Park Wood is a 16.7-hectare (41-acre) Site of Special Scientific Interest in Harefield in the London Borough of Hillingdon. The south-east part is an 8-hectare (20-acre) nature reserve owned and managed by the Hertfordshire and Middlesex Wildlife Trust. The site is ancient woodland which dates back to the Domesday Book. It was part of Harefield Park, which had an eighteenth-century house which later became the site of Harefield Hospital.