O as in …

Obama

Obesity

Object

Obviousness

Ocean

Octopus

Odds

OED

Offender

Offensive

Offer

Office

Oftenness

OH

Oil

OK

Okay

Old

Old Boy

Old Lady

Old Man

Oliver

Olympics

One C

One Shot

One-liner

Onion

Open

Opinion

Opposition

Orange

Orange Soda

Orbit

Order

Organization

Orgasm

Orphan

Orphanage

Osama Bin Laden

Otherness

Otter

Outlet

Outside

Oven

Owl

Oxygen

Oyster

Oz.