A term used for the time period from 1945 through April 25th, 1952 during which the North America and the allied nations of the Second World War 'occupied' Japan.
As Japan needed to rebuild their economy after the war export was encouraged and it was agreed that every second of all exported items should be marked with "Occupied Japan" or "Made in Occupied Japan". Because of this, there might be pairs of pieces with only one of the pair having the OJ mark.
Many of the pieces found with this marking is of porcelain but the pieces ranges from Christmas decorations and toys to pianos. Pieces with this marking are now collected and fakes occur.