Each oak tree is a very metropolis of fairies, and each acorn has its own sprite. Bringing one into the house is a way to enhance contact with the fairy realm. Oak beams are often used to make doors, but the tree itself is a great portal to the other realms.
The oak is associated with many gods all over the world, notably Zeus and Thor. In sacred groves of oak, the Goddess was believed to impart her wisdom through oracles. The oak has sheltered many a king and hero, in myth and real life. The oak spirit is distinct from fairies, and may become very angry if trees are felled or wildlife harmed.
The oak fairy brings courage and a stout heart, necessary to brave the challenges in this world and to journey in the Otherworld. Bearing strength from the heart of the earth, oak fairy can bring steadiness and a deep joy that endures through all.
From: The Fairy Bible