Open interest
The number of option contracts still open in a series after the prior day's clearing. Same-day trades are not reflected until the next update.
Open interest rises when a trade opens new contracts on both sides and falls when both sides close them. It is published by the Options Clearing Corporation once a day, so intraday volume is not yet included.
A day’s volume above the morning’s open interest suggests at least some of the trading opened new positions, but it does not prove that every trade was opening. Comparing open interest across strikes shows where contracts are concentrated, which feeds models such as gamma exposure.
Illustrative example (hypothetical numbers, not a real trade): a put starts the day with open interest of 2,000. During the session 3,000 contracts trade. If the next day’s open interest is 4,100, the net change of 2,100 contracts shows that more positions were opened than closed.