Alcohol is, by far, the most prevalent drug on the OSU campus. The rest of the drug scene isn't terribly huge, and it is kept in check by factors such as price, since students claim that drugs around OSU are more expensive than most places. There are also complaints from certain students about the actual potency of the drugs in the area. Overall, this has been enough to keep it out of the primary social atmosphere. You can find the scene if you're looking for it, but it's not something you're likely to run across by accident (with the possible exception of marijuana).
Marijuana seems to be the drug that always shows up, but to OSU's credit, this happens at nearly every campus. The legalization debate pops up frequently on campus, and "Hemp Fest" takes place each spring on the South Oval (you can get a serious contact high just from walking within a block of it). There are also constant campus flyers for events where you can win free water pipes. While this marijuana subculture isn't officially supported in any way, it is pretty evident; however, it is also quite easy to avoid, and there isn't any social pressure. Stoners tend to band together and not go to class. Outside of the pot crowd, there isn't any readily available or evident drug market, and little to worry about beyond the ever-present alcohol.