Open interest (OI) refers to the total number of outstanding option contracts in a particular options series. This figure increases when new positions are opened, either through a buyer opening a position or a seller writing an option. Open interest decreases when both parties to a transaction close their positions. Where one party opens a position and the other closes an existing position, open interest remains unchanged.
For equity options that expire in-the-money, the total number of underlying shares subject to exercise or assignment can be determined by multiplying the open interest on the expiry date by the contract size.
Updated open interest figures are published by the ASX at the end of each trading day.