Final answer:
OCD is classified under Axis I in the DSM-IV system, which encompasses major mental health disorders; however, the DSM-5 has moved to a single-axis system for diagnoses.
Step-by-step explanation:
Obsessive-Compulsive Disorder (OCD) is classified under Axis I which includes clinical disorders, including major mental disorders, as well as developmental and learning disorders.
The Axis system was used in the DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, fourth edition) to ensure a comprehensive diagnosis that covered all aspects of a patient's condition.
However, it's important to note that the most recent DSM, the DSM-5, has eliminated the multiaxial system in favor of a single-axis system for making diagnoses.