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In the search box, if you click on the magnifying glass you'll get a drop down list showing the places OmniWeb knows how to search. It sounds like you've accidentally selected the "Search This Site" option; to fix, just set it back to "Google".

(Not that it makes much sense for OmniWeb to search for "site:null" when you select "Search This Site" while not actually on a website—that's a bug—but presumably you want to switch your default search to Google anyway.)

Hope this helps!