E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Worn on May 29th. May 29th was the birthday of Charles II. It was in the month of September that he concealed himself in an oak at Boscobel. The battle of Worcester was fought on Wednesday, September 3rd, 1651, and Charles arrived at Whiteladies, about three-quarters of a mile from Boscobel House, early the next morning. He returned to England on his birthday, when the Royalists displayed a branch of oak in allusion to his hiding in an oak-tree.
To sport ones oak. To be not at home to visitors. At the Universities the chambers have two doors, the usual room-door and another made of oak, outside it; when the oak is shut or sported it indicates either that the occupant of the room is out, or that he does not wish to be disturbed by visitors