I went through this same thought process after I heard about the possible connection between previous strep infections and the manifestation of OCD. I thought, "Wow, John had some strep infections when he was younger". But after short consideration, I just decided that it was no use worrying now about how he got the OCD. He has it now and we are dealing with it. You can't go back in your past and try to dredge up old occurrences to bring guilt upon yourself. What's happened has happened, and it's just best to let it go. We know that not all OCD is caused by strep, so you can never be 100% sure that the strep infections caused your son's OCD. Remember, nothing is ever 100% sure. So to give yourself peace of mind, just move forward and focus on the positive aspect of getting your son in the best state of wellness that he can reach. That is a better way to look at it, in my humble opinion.
I haven't read any research about the effectiveness of antibiotic usage for present day OCD symptoms. As a matter of fact, my son was on antibiotics when he first presented OCD symptoms. I immediately thought that the antibiotics had something to do with his anxiety symptoms because they came on so suddenly and strongly. Maybe someone else will have some more info to post for you. Good luck with your son. I understand how you feel.
John 16's Mom