How to read the OI change chart
Open interest tells you where money has been committed, not just where price has been. This guide explains how to read the OI change chart on this site, and what it can and cannot tell you.
- What open interest actually measures
- Why the change matters more than the level
- Reading the chart: green, red, and the dashed line
- Support and resistance from writer positioning
- Choosing a time window
- Four ways to misread it
What open interest actually measures
Open interest is the number of option contracts currently outstanding at a strike. It rises when a buyer and a seller open a new position, and falls when positions are closed. Unlike volume, which counts every trade and resets each day, open interest is a running total of commitment.
The important asymmetry for intraday trading is who is on each side. Option buyers risk a known premium. Option writers — usually institutions — take on open-ended risk in exchange for that premium, and they must defend their position or hedge it. When writing builds heavily at a strike, real capital is committed to price not going through it.
Why the change matters more than the level
A strike may carry large open interest simply because it has been liquid for weeks. What tells you something about today is the change: positions opened or closed in the last few minutes or hours. That is why this site charts the change by default rather than the total, though you can switch to total OI to see where the accumulated walls sit.
| What you see | What it usually means |
|---|---|
| Put OI rising below spot | Put writers building — they need price to stay above; reads as support |
| Call OI rising above spot | Call writers building — they need price to stay below; reads as resistance |
| Put OI falling | Put writers closing or being squeezed — support weakening |
| Call OI falling above spot | Call writers unwinding, often as price approaches; resistance easing |
Reading the chart
Each strike gets a pair of bars: green for puts, red for calls. Bars above the zero line mean open interest was added at that strike; bars below mean it was closed. The dashed vertical line is the current index level, so the question to ask is simply: which colour dominates on each side of that line?
Green stacking up to the left of the line and red stacking to the right is the textbook range-bound picture: writers on both sides are being paid to keep price where it is. Green disappearing below spot while red builds just above it is the opposite — writers are giving up on support and capping upside.
Look at the largest single green bar below spot and the largest red bar above it. Those two strikes are often the day's working range, and the index spends more time between them than outside.
Support and resistance from writer positioning
This is different from support drawn on a price chart. A swing low is price memory: it matters because traders remember it. A strike with heavy put writing is committed capital: somebody loses money if it breaks. The two coinciding is a far stronger signal than either alone, which is why the index options page shows the chart levels and the OI flow side by side.
The summary line beneath the chart does this arithmetic for you: how much put writing was added below spot, and how much call writing above it, over your chosen window.
Choosing a time window
Short windows are noisy; long windows are stale. As a rough guide:
- 5 to 15 minutes — what is happening right now; useful when the index is at a level and you want to see whether writers are defending it
- 30 to 60 minutes — the session's developing structure; best for deciding whether a range is forming
- Full day — where the day's walls were built; the default here, and the right starting point before you zoom in
Four ways to misread it
On expiry, open interest falls across the board as positions settle rather than because anyone changed their view. Unwinding on expiry day is mostly mechanical.
Open interest drops when either side closes. Calls unwinding above spot can mean writers are covering because they expect a move up, or buyers giving up. The price action tells you which.
Large changes at strikes far from spot are often spreads, hedges or rollovers rather than directional bets. Keep the strike window near the money, which is why this page defaults to 20 strikes around spot.
Writers are wrong regularly. Heavy put writing below spot tells you where support is being defended today, not that it will hold.
Putting it together
A useful sequence at any point in the session: open the OI change page on the full day to see where the walls are, switch to 15 minutes to see what is happening now, then open the index options page to see whether the strikes you care about are behaving as that positioning suggests.