No, John O'Callaghan does not do drugs as far as the general public is aware. If he did, it probably wouldn't be very clever of him to reveal it to the world as his reputation as a top trance producer would likely be brought into question.
Many people associate electronic music with drugs, and although it seems a little unfair, there is a reason behind it. The electronic dance scene really took off in the 1990's, and at the time, many people started taking ecstasy at nightclubs. The trend grew rapidly, and before you knew it, the majority of electronic music nightclubs were filled with people that were high on some form of drug.
Is it still the case today?
To an extent, yes. These types of traditions seem to take a long time to subside, and as new fans of electronic music enter the nightclubs for the first time, drug dealers prepare themselves for some easy business.
Of course, with higher security measures and people becoming more aware of the dangers of drug abuse, the numbers of drug using electronic music fans have dwindled. I'm a huge fan of trance music, but I certainly wouldn't condone the use of drugs.
John O'Callaghan is one of the best trance producers around, in my opinion, and it's highly unlikely he would have reached his level of success if he was busy taking drugs.
Here's an interesting look at the popularity of electronic dance music, and the relevance of drugs to its current success: