Obsessive–compulsive disorder (OCD) is characterized by rigid thoughts and behaviors that may compromise cognitive flexibility, yet, especially in adolescence results are inconclusive. This is all the more the case concerning the question whether flexibility deficits emerge specifically when endogenous control, internal monitoring, and working-memory–dependent task-set management are required. In an EEG-beamforming study, we examined adolescents with OCD (N = 68) and healthy controls (HC) (N = 73) in a task-switching paradigm with cue-based (exogenous, externally guided) and memory-based (endogenous, internally guided). Behaviorally, both groups showed typical switch-cost patterns, with greater demands in the memory-based block, and no group differences in switch costs. Oscillatory EEG data revealed marked group differences. During cue-based switching, both groups showed preserved pre-target TBA and ABA modulations in frontal–parietal networks, indicating intact externally preparatory control. In contrast, only HC adolescents showed pre-target TBA increases during memory-based switching, suggesting impaired endogenous task-set updating in OCD. Reactive processes further distinguished groups. In the cue-based block, adolescents with OCD showed switch-related ABA reductions in centro-parietal regions, possibly reflecting inefficient task-set stabilization despite intact behavior. In the memory-based block, HCs displayed coordinated TBA–ABA differentiation supporting internally generated updating, whereas the OCD group showed isolated TBA modulation without accompanying ABA effects, indicating disrupted integration of updating and gating mechanisms. Together, these findings show a dissociation between externally and internally guided flexibility in adolescent OCD.