Realistically, a proportion of people who go through OASC will have tried/used drugs, and I imagine most would lie about it. They take blood/urine samples at the medical and I assume they will test for drug use. However these will only show use in previous 6 months or so. I doubt wether the actual interview notes from the Careers Liaison Officer will be present, but you cannot be certain.
Admitting it in the initial interview was maybe not the best thing to do. However now you've done it, I would recommend repeating it at OASC. The interview is very, repeat VERY fast paced and a short concise answer is what they ask for. They may not even ask if you have personally tried drugs, but they will certainly cover the drugs topic. I would state that I fully understand that drugs and service in the armed forces are totally incompatible etc. If asked wether you personally have tried them, I would say along the lines of...... 'I once tried cannabis at Uni, it made me feel sick, I regret the incident and have never, or will never repeat what was a foolish incident'. I doubt they will press much further on the subject.
In the above post I've used the words.....imagine, assume, doubt, maybe......because you never know quite what OASC are after. However I know for an absolute fact that if you are caught lying/misleading the interviewing officers you will fail selection.
F