Great Britain’s native oaks (pedunculate oak, Quercus robur and sessile oak, Quercus petraea) are currently vulnerable to decline from a number of pests and pathogens, particularly in their southern British range. Stress created by extreme climatic events is likely to increase their susceptibility and this will vary in impact across Great Britain. This Research Note describes the ecological value of Great Britain’s native oaks, as reflected in the biodiversity supported by the trees and ecosystem functions the trees perform. Based on 30 case studies, this Note describes the characteristics of oak woodlands in Great Britain, the consequences of oak decline for oak-associated biodiversity and suggested management actions in preparation for, or in response to, oak decline. Native oak across Great Britain supports 2300 species, of which 326 are obligate (with up to 42 obligate species being associated with certain oak sites). Oak performs ecosystem functions (e.g. leaf litter decomposition) at average rates compared with 16 other tree species that commonly occur or are capable of growing on oak sites. These results suggest that while functioning can be replicated by other tree species, filling the role that oak trees play in providing habitat for associated species is more challenging. Fewer tree species that support oak-associated biodiversity and maintain the ecological function of oak ecosystems occur, or are considered permissible, for planting in native woodlands in Scotland compared with England and Wales.