COBing during the fall quarter at Ohio State is really quite easy. Most (if not all) of the groups will have spaces available since everyone will be under total after graduation this spring. You can just contact the Greek Life office to find out which groups are holding COB events and when/where they will be.
As far as "how" COB works... well, basically you can go to COB events of the groups you're interested in to meet the women. They are usually some time of outing or activity that is similar to what the chapter would do for a sisterhood event. Normally chapters decide fairly soon (based on their own standards) who to offer a bid to. This is of course different from each group and you could end up getting more than one bid in an extreme situation.
ETA... COB in the fall is ONLY for upperclassmen, so you wouldn't be competing with freshmen like you might during formal recruitment in January
