Abstract <p>The relationship between autonomic nervous system function, indexed by heart rate (HR) and heart rate variability (HRV), and the oxidant-antioxidant defense system in obsessive-compulsive disorder (OCD) remains poorly understood. This study investigated the association between autonomic reactivity and salivary antioxidant capacity (AOC) in patients with OCD under cognitive-emotional load. Thirty-one patients with OCD and twenty-nine healthy controls (HC) underwent a cognitive-emotional antisaccade task. Saliva samples for AOC analysis and electrocardiogram (ECG) recordings were collected before and after the task. Participants were stratified into high- and low-HR subgroups based on baseline levels. The OCD group demonstrated significantly lower baseline AOC and a blunted AOC response to the task compared to HC, whose AOC increased significantly. Crucially, stratification by baseline HR revealed a physiologically distinct OCD subtype. OCD patients with low baseline HR exhibited a maladaptive profile, showing a decrease in AOC and a significant increase in normalized low-frequency power (LFnu). In contrast, OCD patients with high baseline HR showed an AOC increase comparable to that of healthy controls. Overall, the OCD group also demonstrated a more pronounced increase in very low-frequency power (VLF) and higher overall LFnu, indicating generalized sympathetic predominance. These findings point to a physiologically distinct OCD phenotype defined by low baseline HR, impaired antioxidant response, and sympathetic hyperreactivity under stress, and suggest that baseline HR may serve as a candidate biomarker of a more severe, maladaptive form of OCD with compromised compensatory mechanisms linking autonomic regulation and antioxidant defense.</p> Clinical trial number <p>Not applicable.</p>