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Go to Yahoo, click in their search field and a magnifying glass icon with a plus next to it will appear in the status bar at the bottom of the window (View>Show Status Bar if you don't already have it in view):

Click the magnifying glass to add the search engine - note the Keyword is the shortcut you can use in the location bar to do a search directly by typing (e.g. for, I use mu).

Select the search engine from the list by clicking the mag glass in the search bar:

OmniWeb will remember the last used engine and use it as the default from then on.