Therapist Guided, Parent-led, Cognitive Behavioural Therapy (CBT) for Preadolescent Children With Obsessive Compulsive Disorder (OCD)

Inclusion Criteria: Children Inclusion criteria: 1. Aged 5 to 12 years old (when parents/carers complete the baseline questionnaires). 2. Diagnosis of OCD (identified from the ADIS-P). 3. UK-resident. Parents Inclusion criteria: 1. Parent/legal guardian of the child. 2. UK resident. Exclusion Criteria: Children Exclusion criteria: 1. Comorbid condition, which is likely to interfere with treatment delivery, for example: 1. Established Autism Spectrum Condition (ASC). 2. Suspected ASC (indicated by exceeding the threshold of 15 on the Social Communication Questionnaire and where a diagnosis of ASC has not been ruled out by a medical professional). 3. Profound learning difficulty (evidenced by attending a specialist school). 2. Risk and/or safeguarding concerns, which are paramount and would interfere with treatment delivery, for example: 1. Suicidal intent. 2. Recurrent or potentially life limiting self-harm. 3. Significant safeguarding concerns (i.e., if the child has a child protection plan and/or is on the child protection register, and/or the research team consider the child to be suffering, or likely to suffer, significant harm). 3. If children have been prescribed psychotropic medication, the dosage must have been stable for two months. 4. Children who are currently receiving other psychological support/interventions delivered by a mental health professional. Parents Exclusion criteria: 1. Significant intellectual impairment that is likely to interfere with treatment delivery. 2. Unable to access or understand written English language materials needed for the intervention.