Background People with contamination-based obsessive‒compulsive disorder (C-OCD) experience contamination-related stimuli as more disgusting than normal, which leads to avoidance behavior or washing and cleaning compulsions. This study aimed to investigate whether evaluative conditioning through repetitive pairing of a contamination-related stimulus with a pleasant stimulus and transcranial stimulation over the orbitofrontal cortex (OFC), the brain area involved in disgust processing, can reduce the intensity of disgust experienced. Method Forty-eight participants (85% women) with C-OCD were included in a four-armed randomized control trial. They received either sham or active transcranial direct current stimulation (tDCS) over the OFC combined with either a sham or active disgust-reduction evaluative conditioning (DREC) program for 10 sessions (five days a week). EEG recordings, self-reported measures, and cognitive tests were performed before, after, and after 2-month follow-up. Results Participants receiving active conditioning showed significant reductions in disgust intensity and obsessive–compulsive symptoms (p ≤ 0.01). Active stimulation was also followed by symptom improvement over time, although no significant additive benefit beyond conditioning alone was observed. Exploratory EEG analyses indicated alterations primarily in theta, alpha, and high-beta activity. Inhibitory control improved in groups receiving active conditioning (p ≤ 0.05), whereas attentional bias remained unchanged. Conclusions The present findings suggest that DREC may reduce disgust intensity and obsessive–compulsive symptoms in contamination-based OCD. OFC-targeted tDCS was well tolerated but showed no significant additive benefit beyond conditioning alone, warranting further study in larger samples. These findings support the potential of disgust-focused interventions in C-OCD.

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