A DO is one step up from a gathering. it's where more than a dozen people get together centered around some event or city or restaurant. It has at least one big dinner with a number of smaller events which could be everything from brunches and tours to even bar crawling
DOS are harder to organize than gatherings because there's money to be collected, restaurants to review, handing several dozen people.
Gathering: 2-12 flyertalkers for a small event like dinner or drinks.
DO: 12 or more people meeting at one central place like a restaurant, with other events during the period of the DO.