Cognitive fusion (CF), involves the tendency to “buy in” to thoughts and feelings and consists of three empirically established domains: somatic concerns, emotion regulation, and negative evaluation. CF is hypothesized to play a role in obsessive-compulsive disorder (OCD). The present study examined how well the CF domains, relative to traditional cognitive-behavioral constructs (i.e., obsessive beliefs such as inflated responsibility), predict OCD symptoms. 52 treatment-seeking adults with OCD completed self-report measures of CF, obsessive beliefs, OCD symptoms, and general distress. Domains of CF were differentially associated with the responsibility for harm, symmetry and unacceptable thoughts OCD dimensions, yet after accounting for obsessive beliefs, only the negative evaluation domain of CF significantly predicted symmetry OCD symptoms. Obsessive beliefs significantly predicted all OCD dimensions except for contamination. These findings provide additional support for existing cognitive-behavioral models of OCD across symptom dimensions, with the exception of contamination symptoms, and suggest that the believability of thoughts and feelings about negative evaluation adds to the explanation of symmetry symptoms. Conceptual and treatment implications, study limitations, and future directions are discussed.