Many words started out as two separate words: maybe (may be), tomorrow, [yesterday](http://www.yourdictionary.com/yesterday), otherwise, and hundreds more, but they are no longer considered compound words. 

As JBJ points out, otherwise comes from Old English [othre wisan](http://www.yourdictionary.com/otherwise): other manner. Some may seen mismatched if we don't know their Old or Middle English roots.