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