O,o
oath
obedient
obey
observe
observe (1-character)
obstacle
obstinate
obstruct
obvious
occasion
occupy
occurrence
ocean
officer
official
offspring
oil
old
old-time
omen
oneself
only
onward
open
open-minded
operate, operation
opinion
opponent
opportunity
oppose, opposite
optimistic
orange
order
order (1-character)
orderly
ordinary
organization
origin
original
ornament
outstanding
outstanding (1-character)
overcome
overflow
owner
oxygen