Oak apple or oak gall is the common name for a large, round, vaguely apple-like gall commonly found on many species of oak. Oak apples range in size from 2 to 4 centimetres in diameter and are caused by chemicals injected by the larva of certain kinds of gall wasp in the family Cynipidae.

Restoration Day, more commonly known as Oak Apple Day or Royal Oak Day, was an English, Welsh and Irish public holiday, observed annually on 29 May, to commemorate the restoration of the Stuart monarchy in May 1660. In some parts of England the day is still celebrated. It has also been known as Shick Shack Day, or Oak and Nettle Day.