Evaulation by a pro. Obsessive thought and compulsive behaviors that result in significant dysfunction is uncommon in it's full form at 5yrs, however people you go on to develop full blown OCD often report very early symptoms. Clearly an evaluation by a good chil;d psychiatrist would be important to determine appropriate treatment. There are many good behavioral and nutritionally based approaches for young children.
R u sure? True OCD in a child is a "big word" that may be found by itself or in association with significant behavioral problems such as autism spectrum disorder. If you think it is ocd, then you should see a child psychologist and a psychiatrist asap.
Therapy is key. Ocd is an anxiety disorder that can respond well to therapy that helps your child rate his level of anxiety in different situations ( e, g. On a " feelings thermometer" so that he can learn techniques to control it. Usually the therapist is a phd or psyd. If you can, find the book " blink, blink, clop, clop. Why do we do things we can't stop".