Okay - I have found 25 towns on the map that begin with the letter "O". They are (in no particular order): Oak Hill, Oakley, Oberlin, Offerie, Ogden, Oketo, Olathe, Overland Park, Olivet, Olmitz, Olpe, Olsburg (done!), Onaga, Oneida, Osage City, Osawatomie, Osborne, Osakaloosa, Oswego, Otis, Ottawa, Overbrook, Oxford, Ozawkie and last but not least, Oak Valley.
If anyone knows of an "O" town I've missed, please let me know - I want to visit them all!!