Go to 'Shortcuts' in the OmniWeb preferences and edit the first one (Shortcut = *, Destination URL = http://www.%@.com). Change the Destination URL to:
http://www.google.com/search?q=%@Now you can type a search term directly in your OmniWeb location bar. OmniWeb will autocomplete it to a valid Google search. If you like to enter more than one serach term at once, use plus (+) to separate them.

