What It Is
OCD involves intrusive obsessions (unwanted thoughts, images, or urges) that cause distress, and compulsions (repetitive behaviors or mental acts) performed to reduce anxiety. OCD and autism share surface features, repetitive behaviors, rigidity, but the underlying mechanisms differ.
How It Presents in Autistic People
In autism, repetitive behaviors are often pleasurable or regulatory. In OCD, they are distressing and ego-dystonic, the person does not want to do them but feels compelled. An autistic person who counts ceiling tiles for comfort is not OCD. An autistic person who counts ceiling tiles because something terrible will happen if they do not is more consistent with OCD.
Treatment and Support
Exposure and response prevention (ERP), a specific type of CBT, effectively reduces compulsive behaviors according to the National Institute of Mental Health. For autistic people it may need adapting, since some triggers may be sensory rather than purely anxiety-based. Look for a clinician with experience in both OCD and autism.