O,o

oath

obedient

obey

observe

observe (1-character)

obstinate

obstruct

obvious

occasion

ocean

officer

offspring

oil

old (1-character)

old-time

omen

oneself

only

onward

open

open-minded

operate, operation

opinion

opponent

opportunity

oppose, opposite

optimistic

orange

order (1-character)

order

orderly

ordinary

organization

origin

original

ornament

outstanding

outstanding (1-character)

overcome

overflow (1-character)

owner

oxygen