As an Admissions Tutor I'd be asking the following questions ;
1. In what way does OCD affect your ability to study? I'd want very specific details/examples and confirming details from whoever diagnosed this and is treating you.
2. Why wasnt this raised either in your application or to the exam boards? I'm suspecting this is only a 'problem' now because you think you may miss your Offer and you are scrabbling about for excuses.
3. If OCD is such a big problem, how will you cope at Uni? Not only with the work but being away from home. If it is such a big problem that you potentially stuffed up your A levels because of it, you need to get this sorted out and under control before you go to Uni. You cant have it both ways - either its a big problem that affects all your studies or it isnt a problem.
4. If you raise this an issue, most Unis will want formal detailed diagnosis reports in order to 'provide appropriate support'. If they see your MH issues as too big an issue for them to cope with (they are a University not a MH Day Centre) then they can and often do either withdraw or defer the Offer until you are better placed to cope with the stress of being at Uni. Remember - if you raise this as an issue, you cannot then stuff the genie back in the bottle if you dont like the consequences of announcing it. Again, is this a real problem - or just ' the University might feel sorry for me' exercise?