Is it the o-ring that itches or the area around?
Anyways...
First, you know that I am going to tell you to get it checked out by a competent medical professional, right? It could be just an irritation, or it could be something as disgusting as pinworms or something. So see a doctor.
You COULD briefly try washing with a mild soap such as Ivory, letting the area dry well, wearing loose cotton underpants, not using premoistened wipes, and applying 1% hydrocortisone cream SPARINGLY to the itchy area twice a day. If these suggestions don't help within ten days or so, show your itching ass to a doctor.
Do you have a cat or dog? Sometimes it may itch when pet hair somehow falls in there.