Tree ecosystems are fundamental to the overall function of the plant, promoting growth and maintaining health, however newly emerging diseases and decline syndromes are increasing in frequency. Current analysis has determined predisposition to disease can be contributed by (a)biotic factors, most commonly affected by drought-stress caused by the increased frequency of drought associated with climate change. Acute oak decline (AOD) is a recently described decline syndrome affecting mature native UK oak trees (Quercus robur and Quercus petraea) greater than 50 years. The specific cause of AOD remains unknown, but Brenneria goodwinii, Gibsiella quercinecans, and Rahnella victoriana have been associated with symptoms and causal agents of the syndrome. This study focuses on characterising the microbiome (rhizosphere) of healthy and AOD-symptomatic oak trees to determine members of the oak microbiome with AOD-suppressive phenotypes. Bacteria across 11 distinct genera were recognised with phenotypic properties suppressing the growth of AOD pathogenic strains of B. goodwinni, G. quercinecans, and R. victoriana, and identified through bacterial 16S rRNA gene sequencing. Comparison of three major compartments of Q. robur using location data of sampled bacteria (foliage, rhizosphere, and stem) associated bacterial 16S rRNA gene sequences with individual compartments, suggesting which major compartment of Q. robur has the most AOD-suppressive properties. Bacteria suppressing G. quercinecans growth was most abundant in the foliage and rhizosphere, composed of 72.27% and 72.70% respectively, of total successfully sequenced bacteria; bacteria suppressing R. victoriana growth was most abundant in the stem, composed of 77.78% of total successfully sequenced bacteria.