Oak (Plant) - Oak is a symbol of power and strength, and is a "wand tree". Hagrid's wand is made from oak.

Obliviate (Spell) - to erase the victim's memory.

Occamy - from India and the far east, an odd creature with a plumed, serpentine body, two legs and wings. It can grow to fifteen feet and is an aggressive defender of its eggs, the shells of which are pure silver.

Occlumency - the use of magic to defend one's mind against external invasion

Oinomancy - practiced by ancient Romans, this form of Divination used sediment left in the bottom of a wine cup to interpret the future.

Oneiromancy - Divination by dreams. The Greek word oneiros means "dream" plus mancy means "prophecy".

Oppugno (Spell) - Latin for "to attack or assault". A spell which uses conjured creatures to attack another.

Orchideus (Spell) - a bunch of flowers bursting from the tip of a wand.

Ornithomancy (Divination) - the study of the activity of birds.

Ouija (Divination) - a board with the alphabet on it, used with a planchette to spell out supernatural messages.

Owl - a nocturnal bird of prey possessing a hawk-like beak, claws and large head with front-facing, round eyes.

The owl has been considered both good and bad throughout history. The screech of the owl is considered to be an ill omen in some parts of the world, perhaps a portent of death. Since they are awake during the night, an association was formed with magic. Medieval wizards relied upon owls for their powers of observation, and for their ability to memorize long, complicated spells and formulas.

Older cultures also admired the owl. Athens, for instance, had the owl as its emblem. Since Athens was a major center of learning in the ancient world, owls came to symbolize intelligence. They became the emblem of the Roman goddess of wisdom, Minerva, after whom Professor McGonagall was named.

Owls prove their natural intelligence in the wizard world every day and night, since they unerringly deliver post to the intended recipients of the many letters being sent.