There are standardized instruments to assist with the diagnosis of OCD and related disorders and there are instruments used to assess symptom severity.
OCD and other psychiatric disorders can be reliability diagnosed using the Structured Clinical Interview for DSM (SCID). This is a clinician-administered instrument that may take up to 90 minutes if screening for all psychiatric disorders, but it provides information on both current and lifetime disorders. However, the module for OCD can be used on its own in clinical practice to make a reliable and valid diagnosis of OCD.
The Mini-International Neuropsychiatric Interview can also be used to diagnose OCD and certain other psychiatric disorders. The instrument mostly assesses current disorders, not lifetime. The OCD questions could be used on their own to help with the diagnosis.
The Yale-Brown Obsessive Compulsive Scale Symptom Checklist is not a diagnostic instrument but once the diagnosis of OCD is made, this instrument can provide the clinician with a thorough assessment of every obsessive thought and compulsion that the person has or had in the past.