Options volume vs. open interest
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Volume counts contracts traded during a period. Open interest counts outstanding contracts that have not been closed, exercised or expired. Trading volume and a change in open interest measure different things and should be compared using their respective timestamps.
Activity is different from outstanding positions
A contract can show today's volume of 200 and open interest of 500 from the latest completed clearing cycle. The 200 measures trading during the session; the 500 measures outstanding contracts at its stated time. Neither figure identifies who traded or how many contracts were bought to open.
How opening and closing affect open interest
For a one-contract trade, a new buyer and a new seller opening positions increase open interest by 1. Two participants closing positions decrease it by 1. If one opens and the other closes, it is unchanged. All three cases add 1 to volume, so 200 contracts of volume do not automatically add 200 to open interest.
Volume is only one liquidity check
An active stock can still have a wide spread on a particular expiration or strike. Before placing an order, inspect the current bid, ask, quoted size, open interest and underlying price in your broker's live interface. A daily stock-level volume ranking does not guarantee that every contract is liquid or executable at the last traded price.
Compare timestamps before interpreting a change
Start with the exact contract, including expiration, call/put and strike. Note the volume session and open-interest date, then compare the next published open-interest figure if available. A change from 500 to 550 is a net increase of 50 outstanding contracts, not proof that 50 institutions bought or that prices will rise. Exercises, expirations and corrections also matter.