Otto comes to mind.
Oliver Ollie Oli Oscar
over and under
odalisque
oriole
orlop
osier
Orc, Olog-hai
Obediah?!
Oscarโs, Oliver, Omar, Otto, Olaf, Orion, Oakley
A few examples of 3-letter words starting with the letter O are:owloweoaroatown
Some six letter words starting with O are:objectoblongoccultocelotoddityoffendofficeorangeorioleorphanoverdooxygen
Old Navy and OshKosh B'Gosh are clothing brands. They begin with the letter O.