Obsessive-compulsive disorder (OCD) and eating disorders (ED) are compulsive disorders with overlapping symptoms. However, weight loss and over-exercise causing secondary medical complications are rarely seen in OCD. We report the case of a 15-year-old male who presented with atypical symptoms of OCD leading to severe medical compromise. Covid-19 related team sport restrictions led to compulsive exercise associated with intrusive thoughts. The onset of stress fractures limited exercise ability, prompting compensatory food restriction. Bradycardia, hypothermia and hypoglycaemia resulted from severe malnourishment and weight loss in the context of OCD. His weight was 85.8% of ideal body weight on admission, reflective of a weight 10–15 kg lower than his premorbid weight. During admission, he developed exercise-induced rhabdomyolysis with significantly elevated creatinine kinase and required intensive care unit treatment. Psychotropic medication included lamotrigine, olanzapine and high dose fluoxetine alongside cognitive-behavioural therapy. Medical stabilisation and weight restoration allowed discharge to an outpatient Child and Adolescent Mental Health Service. This atypical case of OCD highlights the potentially life-threatening risks associated with excessive exercise and malnutrition. This paper highlights the complexities of treatment in a patient who cannot adhere to bed rest and the differential diagnoses of anorexia nervosa, orthorexia nervosa and exercise addiction.