Open Interest

Most descriptions of open interest start with what it signals. Start instead with the mechanism, because once you have it the signals are obvious rather than memorised.

Volume resets every session and counts transactions. Open interest is a running total of contracts that exist — positions opened and not yet closed. It only changes when the reason for a trade changes it.

Every trade has a buyer and a seller, and each of them is either opening a new position or closing an existing one. Four combinations, and they do three different things:

Exhibit 1 · The four trade combinationsWhat each does to the count

So the useful reading is never open interest alone. It is open interest against volume and price. A day with heavy volume and unchanged open interest was a day of people swapping positions with each other. A day with the same volume and a large rise in open interest brought new money into the market.

One consequence people find counter-intuitive: for every long there is exactly one short, always. Open interest is not a measure of bullishness or bearishness — it is a measure of how much disagreement has been financed.

Pair the direction of price with the direction of open interest and you get four readings. Unlike most such tables this one is mechanical rather than interpretive — it follows directly from section 01.

Exhibit 2 · Price against open interestWho is doing the moving

A rally on rising open interest is being bought. A rally on falling open interest is being escaped. They look identical on a price chart and almost nothing about them is the same.

All four states in sequence, which is how they usually arrive.

Exhibit 3 · Build, crowd, flush, squeezePrice · Volume · Open interest

The crowding phase is the one to learn

Look at the stretch where price goes sideways and open interest keeps rising. Nothing is happening in price, so it attracts no attention — and it is the most informative part of the chart. Positions are accumulating without price rewarding them, which means a growing population is sitting on a trade that is not working.

That is a loaded spring, and it is the setup for both of the moves that follow. The people who financed that disagreement have to resolve it eventually, and when they do the resolution is fast, because it is made of closing orders that are indifferent to price.

The same word, a related mechanism, and a meaningfully different use — worth separating rather than blurring.

In futures, open interest is one number per contract month and tells you how crowded the trade is. In options it is one number per strike and expiry, so what you get is a distribution across prices — closer in spirit to a volume profile than to a futures reading.

Two things that distribution is genuinely useful for:

1. Locating the crowded prices. Large open interest at a strike means a large number of contracts whose value changes character at that price. It marks where a lot of people have agreed something matters.
2. Understanding who has to hedge. Dealers who sold those options manage their risk by trading the underlying, and how much they must trade changes as price approaches a heavily-populated strike. That is a real, mechanical flow, and it is why price sometimes behaves oddly near big strikes into an expiry.

The honest caveat: the popular retail versions of this — "max pain", simple put/call ratios — compress a distribution into one number and lose most of what made it informative. The direction of dealer hedging depends on who is long and short the options, which open interest alone does not tell you. Treat the single-number versions as curiosities.

• Equities do not have it. Shares are not contracts that get created and retired, so there is no open interest on a stock. This instrument exists for futures and options only, which is most of the reason it is under-used — most people never trade something that reports it.
• It arrives late. Exchange open interest is typically published the following morning, so you are reading yesterday's positioning. Positioning surveys are slower still — the CFTC's weekly commitments report is compiled from a Tuesday and released on a Friday.
• Rolls distort it. As a contract approaches expiry, open interest collapses in the front month and appears in the next one. That is administrative, not a change of conviction, and it wrecks any reading taken across a roll. Use continuous or aggregate open interest across months.
• It does not tell you who. A rise could be directional speculators, a producer hedging output, or one leg of a spread with no directional view at all. Positioning reports help and are coarse.
• It has no direction of its own. Every contract has a long and a short. Open interest measures the size of the disagreement, never which side of it is right.

• Always read it with price and volume, never alone. The three together identify the state; any one of them alone identifies nothing.
• Ask one question: is this move new positioning or unwinding? Rising open interest means arriving; falling means leaving. Moves made of leaving are fast and finite.
• Watch for price flat and open interest rising. The crowding signature, and the most informative configuration on the chart precisely because nothing is happening in price.
• Treat a sharp fall in open interest as an ending, not a beginning. Whether it comes with a rally or a break, it means a population has finished, and finished populations do not come back.
• Use continuous or aggregate series. Front-month open interest across a roll is measuring the calendar, not the market.
• For options, read the distribution, not a ratio. Where the contracts sit across strikes is the information; compressing it to a single number throws that away.

A description of market mechanics, not trading advice.