Linn-
The feature you want is not currently implemented in OO3. It's something we're planning to add for OO4. However, in the meantime, we do have an AppleScript up on our website in the useful scripts package that will do this for you.
http://www.omnigroup.com/ftp/pub/sof...fulScripts.dmg
Instructions to install it are included.
Look for the script 'Collapse to Level X'.
Let me know if you have any questions.
--
Derek M.
Support Ninja
The Omni Group