You need RobTrew's Where in OF script, which is a veritable Swiss army knife of searching. You can currently find it here:
http://www.complexpoint.macmate.me/S..._by_Query.html
The canonical location is
http://bit.ly/OF-Find, but that doesn't seem to have been updated to point at the new location since Apple pulled the plug on MobileMe.
A FAQ on installing and using Applescripts with Omni products can be found here:
http://forums.omnigroup.com/showthread.php?t=7453