Hundreds of 13
letter words start with the letter O. A few of those words include
objectionable, objectiveness, obnoxiousness, observational,
obsolescently, obtainability, offensiveness, officeholders,
openheartedly, operationally, opportunistic, opportunities,
orchestrating, organizations, outdistancing, outmaneuvered,
outperforming, outrebounding, overabundance, overachievers,
overcommitted, overconfident, overdeveloped, overestimates,
overpopulated, overprescribe and overregulated.