What Is Open Interest in Options Trading?

Open interest is the number of option contracts that are still open. In plain English, it tells you how many contracts are alive and haven’t been closed out yet.

When both sides of a trade open new positions, open interest goes up. When both sides close, open interest goes down. Volume tells you how much traded today. Open interest tells you how much is still on the board.

An easier way to think about it

A good metaphor is restaurant reservations:

• Volume is how many people walked through the door today.
• Open interest is how many tables are still booked.

A restaurant with high volume but low open interest had a busy day with people coming and going. A restaurant with high open interest had a lot of standing commitments on the books.

Why it matters for gamma analysis

Gamma exposure is built on options positioning, and open interest is one clue about where that positioning lives. If a strike has large open interest, it may matter more than a strike nobody cares about. Big positions need to be hedged, and the hedging activity around them is what creates structural levels on the chart.

But don’t overcomplicate it:

• High volume means activity today.
• High open interest means existing positions are already there.
• Both together can tell a stronger story than either one alone.

The catch every trader needs to know

Open interest is not a real-time number. The exchanges publish it once per day, after the close. The OI you see at 10am today is the same number from last night’s close — it will not update again until tomorrow morning.

This is fine if you’re trading positions you plan to hold overnight. It’s a serious problem if you’re trading intraday — because more than half of modern options volume happens in 0DTE contracts that open and close inside the same session and never touch end-of-day OI at all.

Any tool that calculates gamma exposure from daily OI snapshots is structurally blind to that intraday flow. It’s showing you yesterday’s map of dealer positioning while half the action is happening off the map.

How to use it

Open interest is great background structure. It tells you where the slow-moving money sits — the positioning that survived the close and likely still matters. Use it as context, not as a live heartbeat monitor.

For intraday trading, you need something more responsive. That’s why GammaFlux combines OI with live options chain data and live futures volume to estimate where dealer positioning is shifting in real time, not just where it sat at yesterday’s close.