Which form fits depends on your question and the size of your group. Below are the three I use, and when I advise against each.

Open Space Conversation Café World Café
Group size 20 to 1,000+ 5 to 12 per table 12 to 100
Who sets the agenda the participants no one, one theme the organiser
Strongest for complex challenges, many stakeholders a conversation that needs depth deepening pre-chosen themes
Produces self-chosen sessions with a harvest shared understanding insight per theme
Advised against when the outcome is decided, or the group is too small a decision has to be made participants want to ask their own questions

For groups under twenty I usually choose Conversation Café. Sometimes as a run-up to an Open Space later on: first understand together what's going on, then get to work self-organising.

Conversation Café started in the summer of 2001 in Seattle, created by Susan Partnow, Habib Rose and Vicki Robin. The form is simple: a small circle, a talking object, a few rounds, ninety minutes. Whoever later took over stewardship came from the Open Space tradition themselves, which explains why the two forms fit together so well.

None of these three fit your question? See when Open Space is not the answer.

For a fuller treatment of this choice: Choosing Open Space Technology by Birgitt Bolton, on openspaceworld.org.