Open interest (OI) in an options series is the total number of outstanding contracts in that series. Open interest increases when a buyer buys to open or a seller sells to open. Open interest falls if both buyer and seller close their positions. It remains the same if one party to a transaction is opening a position while the other is closing theirs.
If an equity option is in the money, you can calculate the number of underlying shares that will be exercised or assigned by multiplying the open interest on the expiry date by the contract size.
The ASX releases new figures for open interest at the end of each trading day.