Background Impaired cognitive control processes may be central in the pathogenesis of obsessive‐compulsive disorder (OCD). Our objective was to evaluate cognitive control processes with event‐related potentials in early‐onset OCD (EO) and late‐onset OCD (LO), which are suggested to have distinct characteristics. Methods Participants were unmedicated EO (n = 26) and LO patients (n = 33) without comorbid psychopathology and healthy controls (n = 54). Go/No‐go tasks with 50 and 80% Go trial probabilities were implemented to manipulate the strength of response conflict and inhibitory demands. Results LO patients had shorter N2 latencies than controls and did not show the N2 amplitude increase seen in controls with the increase in Go trial probability as suggestive of abnormal conflict monitoring processes. Both EO and LO patients showed smaller P3 increase than controls with the increase in Go trial probability, suggesting problems in modifying attentional control with changes in task demands. P3 was more anteriorly distributed in LO patients than controls. Additionally, P3 increase, with the increase in Go trial probability, was larger in frontal and central sites than in parietal sites in controls, whereas in EO patients it was almost homogenous across anteroposterior sites. Conclusions N2 processes were affected only in LO, whereas P3 processes were affected in both EO and LO, although, somewhat differently. P3 distributions suggest that EO and LO patients have differences concerning the contributions of frontal and parietal components of attentional networks to the execution of Go/No‐go tasks. Our results imply that EO and LO are distinct subtypes affecting the cognitive control systems differently.