Objective The aim of the present study was to investigate the influence of comorbid oppositional defiant disorder (ODD) on clinical features and neuropsychological profiles of children with attention-deficit/hyperactivity disorder (ADHD). Methods We divided the participants into three groups: the ADHD with ODD (ADHD/ODD) (n=36), ADHD without ODD (ADHD/noODD) (n=307), and control groups (n=128). Parents of the participants completed the ADHD Rating Scale, Social Responsiveness Scale (SRS), Korean Personality Rating Scale for Children (K-PRC), and 10-item mania scale from the Parent General Behavior Inventory (P-GBI-10M). Neuropsychological profiles were assessed using the Advanced Test of Attention (ATA), Children’s Color Trails Test, and Stroop Color and Word Test. Results The ADHD/ODD group had more ADHD symptoms and functional impairments in relationships with teachers and peers, and self-esteem than the ADHD/noODD group. The ADHD/ODD group scored higher in Social Communication (p<0.001) and Autistic Mannerisms (p<0.001) subscales of SRS, P-GBI-10M (p<0.001), and Delinquency (p<0.001) and Psychosis (p<0.001) subscales of K-PRC than the ADHD/noODD group. Commission Errors (p<0.001) and Response-Time Variability (p<0.001) in Visual ATA and Commission Errors (p<0.001) in Auditory ATA were significantly higher in the ADHD/ODD group than in the ADHD/noODD group. Conclusion The present study suggests that patients with ADHD with ODD experience more ADHD symptoms and neuropsychological deficits than those with ADHD without ODD. These results also imply that comorbid ODD is associated with greater social impairment and emotional dysregulation.