Obeisance
A gesture of deference (bow curtsy)
Obdurate
Unyielding inflexible
Obfuscate
To make confusing
Obsequious
Fawning attentiveness
Obstinate
Stubborn
Obstreperous
Noisy
Obtain
Accepted established
Obtuse
Not clear or precise lacking
Obviate
To anticipate and make unnecessary
Occlude
To obstruct or block
Occult
Hidden
Odious
Hateful
Officous
Pushy and in offering services where they are unwanted
Onerous
Troubling burdensome
Opaque
Dense in mind
Opprobrium
Disgrace scorn
Oscillation
Swinging back and forth
Ossifed
Changing into bone
Ostensible
Seeming appearing as such
Ostentatious
Designed to impress or attract notice
Overweening
Overbearing