Obsessive Compulsive Disorder (OCD) is among the most common psychiatric comorbidities with eating disorders (EDs) and most studies have only examined this relationship at a diagnostic level. More research is needed to determine whether specific symptom domains and cognitive patterns commonly observed in OCD are most salient among individuals with clinically significant EDs, and whether these symptoms appear to change and/or influence treatment outcomes. Thought Action Fusion (TAF) is one cognitive pattern that may underlie OCD-ED comorbidity. The current study assessed 112 adolescent and adult female patients at a residential ED treatment facility on levels of ED severity, OCD symptom severity, and TAF at pre- and post-treatment. All OCD symptom dimensions were positively correlated with ED severity at pretreatment, with Obsessing, Neutralizing and Ordering OCD symptoms being most elevated. TAF was also positively correlated with ED severity at pre-treatment, and higher levels of TAF at pretreatment significantly predicted greater ED severity at post-treatment after controlling for all other OCD symptoms. Improvements in TAF specific to thoughts about others also predicted improvements in ED severity after controlling for changes in OCD symptoms. Clinically, these results indicate that efforts targeting specific OCD symptom dimensions and TAF in addition to ED-focused treatment as usual may be beneficial for enhancing overall treatment outcomes.