Open interest vs volume in a GEX chart

Two numbers sit beside nearly every option contract: open interest and volume. They look similar enough to invite confusion. The difference matters whenever someone uses an option chain to infer dealer gamma, a call wall, or a possible hedge flow.

Volume counts contracts traded during the current session. Open interest counts contracts still open after positions have been processed through clearing. Volume is activity; open interest is a stock of outstanding contracts. A busy tape can leave the stock almost unchanged.

One trade, several possible effects

Imagine 10,000 contracts trade at one strike. If buyers and sellers are both opening new positions, open interest can increase. If both sides close existing positions, it can decrease. If one side opens while the other closes, the trade can change who holds risk without changing the total open count. A volume print by itself does not identify which case occurred.

The identity of the counterparty matters just as much. The public chain does not show whether a market maker took the other side, whether the position was offset with another option, or whether it was hedged in futures or shares. That is why the phrase “dealers must buy” deserves suspicion when it is based only on a large volume bar.

Why yesterday can still be on today’s screen

Standard open interest is published after clearing, while prices keep moving during the next session. That timing gap matters when reading any intraday options chart. A changing display should never be mistaken for a live census of outstanding positions.

That is often enough to make the chart useful as context. Trouble starts when a screen’s refresh rate is mistaken for proof that the position estimate is equally fresh. Check whether the provider describes its numbers as open interest, estimated flow, or a combination.

What the two numbers are good for

Open interest helps locate where existing option commitments are concentrated across strikes and expiries. Volume shows where attention and trading activity have arrived today. A strike with large open interest and little volume may still affect a model because the outstanding book is large. A strike with little prior open interest and a burst of volume may deserve a second look, though the direction of its net position remains uncertain.

Suppose a call strike carried heavy open interest overnight, then saw unusually large volume before noon. The sensible response is to watch the next official open-interest update and inspect how price behaves near the strike now. Assuming every contract traded today added to the overnight position would double count risk that may have been closed or transferred.

A better question for any GEX provider

Ask which figures are published market data, which are estimates, and when the chart was last updated. Two tools can show different net gamma even when they cover the same market because their interpretations differ.

No single public number settles the dealer book. Open interest and volume are both valuable when each is asked to answer the question it actually measures.