O,o
oath
obedient
obey
obscure
observe
observe (1-character)
obstacle
obstinate
obstruct
obtain
obvious
occasion
occupy
occurrence
ocean
offend
offer
office
officer
official
offspring
oil
old
old-time
omen
oneself
only
onward
open
open-minded
operate, operation
opinion
opponent
opportunity
oppose, opposite
optical
optimistic
optimum
orange
order
order (1-character)
orderly
ordinary
organ
organization
origin
original
ornament
other, another
out
outline
outspoken
outstanding
outstanding (1-character)
over
overcome
overflow
owner
oxygen