The overall BOS works the same was as the mini-BOS that happens within a category when there are multiple judging teams working a category. At the DMC, for the overall BOS they round up the first place finishing beer from each category (that is one reason why you send 2 bottles) along with the top ranked judges. They pour the beers, and the judges basically rank them 1st, 2nd, and 3rd. There are no score sheets, and none of the scores from the earlier rounds get carried forward. It is similar to a dog show where after the individual breeds get judged, the rest just prance around the ring for "best in show" until the judges pick out the 3 that they feel are the best example of their style compared to all the others. You will not know if your beer was the 4th best beer in the BOS, or if it was the 15th best beer.
When you get your score sheets in the mail (possibly as soon as today unless the Chicago post office looses the envelope), you should have some very good comments, and in the case of any winnnig beer(s) you may actually see two scores on the cover sheet. One will be crossed out. The way the DMC handles things, and the way many other comps have started handling things, is that the initial score from the judges is written on the cover sheet. Then, after the mini-BOS judging (if there is one), they will "assign" a score to give the top three beers a higher score than the "4th place" beer. This is indicated by crossing out the initial score, and writing a new score on the cover sheet. They do this simply because the scores for the mini-BOS may have come from two or more different judge teams initially and it would be possible that the 1st place beer after the mini-BOS ended up with a lower score than the 3rd place beer. By assigning the score, it cuts down on the number of questions that the organizers get when two friends compare scores after the fact and a beer that did not place ends up with a higher score than one that did place.