The aim of this study is to use a voxel-based morphometry protocol to compare the brains of 18 children with obsessive-compulsive disorder (OCD) with those of a healthy group matched for gender and handedness. Images were acquired with a 1.5-T MRI scanner, spatially normalized, and segmented with an optimized voxel-based morphometry protocol. OCD children presented a 5.93% reduction of gray matter (GM) total volume in comparison with control brains. We identified OCD brain volume reductions in regions that have been extensively related to action monitoring and error signaling processes. Specifically, we found decreased bilateral GM in frontal (significant after Family Wise Error (FEW), multiple comparisons correction) and cingulate regions as well as decreased white matter (WM) in bilateral frontal and right parietal (p<0.001 uncorrected). Additionally, we found a negative correlation between symptom severity and bilateral hippocampal GM-volume (p<0.001uncorrected) as well as a positive correlation between age and GM left caudate volume (p=0.037 FWE small volume corrected) in the OCD group. As a conclusion, our results point to conflict monitoring structural brain regions as primary deficits in pediatric OCD, and to striatal abnormalities as age-related deficits.