Old Park Wood is a 16.7-hectare (41-acre) Site of Special Scientific Interest in Harefield in the London Borough of Hillingdon.

[3] Its highlight is the abundance of flowers in spring, with a carpet of bluebells together with yellow archangel, lesser celandine, wood anemone and the rare coral root bittercress (Cardamine bulbifera).

Golden saxifrage and marsh marigolds grow along small streams and there is a pond which is important for dragonflies and invertebrates.

There is no access from the hospital, but a footpath along its border fence from Hill End Road leads to the Herts & Middlesex nature reserve.

Another footpath between Summerhouse Lane and Hill End Road goes through the SSSI, skirting the northern boundary of the nature reserve.