O as in …

Obama

Obesity

Object

Obviousness

Occupation

Ocean

Octopus

Odds

OED

Offender

Offensive

Offer

Office

Oftenness

OH

Oil

OK

Okay

Old

Old Boy

Old Lady

Old Man

Old Woman

Oliver

Olympics

One C

One Shot

One-liner

Onion

Open

Operations

Opinion

Opposition

Optimist

Oral

Oral Sex

Orange

Orange Soda

Orbit

Order

Organization

Orgasm

Original

Orphan

Orphanage

Osama Bin Laden

Otherness

Otter

Outlet

Outside

Oven

Overpopulation

Overweight

Owl

Owner

Oxygen

Oyster

Oz.