"short" can be relative, of course. search OpenSpaceTechnology/OSLIST archives for Harrison Owen adn Michael Pannwitz' Berlin OS Day4 event with 2108 people. Search there for "2108" That was one day of OS after three traditional conference days.
in another situation, Larry Peterson suggested the following, with only two hours available to open, meet, close and close the conference:
What is the size of the group? This makes some difference to me in terms of design. If you really have two hours to both explore possible next steps and close the event then I make this suggestion:
Do one round of people identifying the next steps for which they have a passion, and then have them lead a 45 min or so of discussion making flip chart reports. I would facilitate the volunteering of people and topics on flip chart and then assign them a physical space if the group is of some size (I’ve done this with groups up to 100 or so). If it is a group of 25-30 then posting topics and a regular opening can work in 30 minutes.
A brief 1-min report back to give a flavour before the closing circle if often needed in these session for some sense of “convergence”. You would then have 30 min or so for a closing exercise, with the talking piece. This could close the whole event.
I do not call it “Open Space Technology” but a taste of what the process can do. I don’t think the “space” is really opened without the chaos at the wall and a couple of rounds of choices. When it is one round, it is still clear that the facilitator is giving direction even though the small group discussions will be self-organizing. Self-organizing small group discussions based on volunteer leadership are a part of many good facilitation processes and can be energizing and productive. I just don’t call it Open Space if it is one round and the topic generation is more guided because of the time.
That’s my experience and advice.