Open Interest vs. Volume: What Each One Actually Tells You
Two numbers, two clocks
Every strike on an option chain carries two numbers that look interchangeable but run on completely different clocks. Volume resets to zero at the open and counts every contract traded that session, regardless of whether the trade opened a new position or closed an old one. Open interest doesn't reset at all — it's a running total of contracts still outstanding, published once per day after the exchange's overnight clearing process. A strike can show heavy volume with almost no change in open interest (existing holders trading with each other), or it can carry enormous open interest with zero volume today — an old position nobody is touching. The two numbers answer different questions: volume asks "how active is this contract right now," open interest asks "how much is actually parked here."
What actually moves open interest
Every trade has two sides, and each side is independently either opening a new position or closing an existing one. That gives four combinations: open buys to open sells (open interest rises by the traded size), close sells to close buys (open interest falls), and two mixed cases — an opening trade matched against a closing trade — where open interest doesn't move at all even though volume ticks up. This is why a busy contract can show flat open interest day over day: a lot of same-day traders are opening and closing the identical strike, netting out to no change in the standing total once the day's clearing runs.
Reading an open-interest ladder across strikes
Look at open interest across an entire expiration rather than one strike and a shape usually emerges — concentrated pockets at round numbers, recent earnings-move targets, or wherever large funds have parked hedges. That concentration matters because market makers who sold those contracts typically hedge their own exposure in the underlying stock, and the size of that hedge changes as price moves relative to the strike. Heavy open interest sitting just above or below spot is one of the inputs behind "gamma exposure" style analysis — the idea that dealer hedging flow can act as a magnet or an accelerant near strikes where a lot of contracts are concentrated. Open interest alone doesn't tell you which side initiated a position or why it's there — a strike can carry huge open interest from hedges, spread legs, or plain market-maker inventory that has nothing to do with a directional bet — so treat the ladder as a map of where size sits, not a prediction of where price goes.
When volume outruns open interest
If a contract trades more volume in a single session than its entire standing open interest, the math forces a conclusion: there weren't enough existing contracts for all of that volume to be closing trades, so a large share of it has to be brand-new positioning. That's the simplest version of the Volume > OI screen traders use to flag unusual activity, and it can show up a few different ways under the hood — a single large block trade negotiated off the order book, a sweep order that hits multiple exchanges' resting liquidity at once because someone wants a fill fast, or just a wave of smaller retail orders piling into the same strike after a headline. All three look identical in the raw volume number; distinguishing them requires looking at trade size distribution and whether the flow hit the bid or the ask, not just the Vol>OI flag on its own.
Open interest as a liquidity signal, not just an activity flag
Beyond flagging unusual activity, open interest is a decent proxy for how easy a contract will be to trade later. Strikes with deep, established open interest tend to attract tighter market-maker quotes because there's a known pool of existing positions likely to trade against new orders; thin, low-OI strikes — often far out-of-the-money or in a distant, illiquid expiration — can show wide bid-ask spreads even when the underlying stock itself trades a huge daily volume. Before opening a position you plan to exit before expiration, checking open interest at your specific strike is at least as useful as checking the stock's overall option volume. See the hidden cost of a wide bid-ask spread for how that liquidity gap shows up in your actual fill price.
You can see real, live open interest and volume — with the Vol>OI flag applied automatically — on any symbol's option chain in OptionScope.