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Open interest is the number of option contracts currently outstanding — positions that have been opened and not yet closed, exercised, or expired. Every contract has one long side and one short side, and open interest counts the pair once.

It rises when both parties to a trade are opening new positions, falls when both are closing, and stays flat when a contract simply changes hands. That makes it a running tally of positioning, where volume is a tally of activity.

Open interest updates once per day, after the clearing process, so today's trading shows up in tomorrow morning's figure.

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The mechanics are easiest to see in sequence. Take one contract in a quiet stock:

• Monday: trader A buys 10 contracts to open and trader B sells 10 to open. Volume prints 10, and open interest rises from 0 to 10 overnight.
• Tuesday: A sells the 10 to close and trader C buys 10 to open. Volume prints 10 again, but open interest stays at 10 — the contracts changed hands without changing the total outstanding.
• Wednesday: C sells 10 to close and B buys 10 to close. Volume prints 10, and open interest returns to 0 the next morning.

Three identical volume prints, three different effects on open interest. That is why the two figures are read together: volume says how much traded, and the next day's change in open interest says how much of it was net new positioning.

Open interest also has a practical side. Strikes with large open interest tend to have more competing quotes and tighter markets, which is part of what liquidity looks like on a chain. Concentrations often build at round-number strikes and near-dated expirations, and where they sit across the market is visible on the activity page.

El detalle formal

Formally, open interest is the count of contracts recorded as open at the clearing corporation after each session's settlement. Every contract pairs one long with one short, and the pair counts once — open interest of 10,000 means 10,000 contracts exist, not 10,000 participants or 20,000 positions.

Its limitations follow from how it is produced. It updates once daily, so it always describes yesterday. And a day's change is the net of all opening and closing activity, so a flat print can conceal heavy two-way repositioning underneath.

The persistent misconception is reading open interest as directional. A protective put bought against stock, a put sold to collect premium, and the short leg of a spread all add identical open interest; the figure says positions exist, not why they exist. Nor is open interest money waiting to move — both sides of every open contract already paid or collected their premium when the trade printed. Large open interest is best read as evidence of participation and hedging activity at a strike, nothing more specific than that.