Background: We conducted a comprehensive and systematic assessment of memory functioning in drug-naive boys with attention deficit hyperactivity disorder (ADHD) and oppositional defiant disorder (ODD). Methods: Boys performed verbal and spatial working memory (WM) component (storage and central executive) and verbal and spatial storage load tasks, and the spatial span, spatial executive WM, spatial recognition memory and verbal recognition memory tasks from the Cambridge Neuropsychological Test Automated Battery. Groups comprised: (a) ADHD only (N = 21); (b) ADHD+ODD (N = 27); (c) ODD only (N = 21); and (d) typically developing (TYP) boys (N = 26). Groups were matched for age (M = 9.7 years) and sex (all boys). Results: Confirmatory factor analyses confirmed the presence of five factors: verbal functioning, spatial functioning, WM storage, WM central executive and long-term memory (LTM). All three clinical groups demonstrated impaired memory performance. Boys with ODD and ODD+ADHD but not ADHD alone performed poorly on verbal memory tasks, whilst all three clinical groups showed impaired performance on spatial memory tasks. All three clinical groups performed poorly on the storage and central executive WM factors and the LTM factor. Conclusions: ADHD and ODD are characterised by impaired performance storage and central executive WM tasks and LTM tasks. This is, we believe, the first report of impaired WM and LTM performance in ODD. This study suggests that verbal memory difficulties are more closely associated with ODD than ADHD symptoms and that combined ADHD+ODD represents a true comorbidity. The data also support a small but growing number of suggestions in the literature of impaired LTM in ADHD.