Open interest counts open positions. When a buyer and a seller transact in a new contract, open interest rises by one. When they close that contract, open interest falls by one. Unlike volume, which counts every transaction, open interest is a stock-not-flow measure of how much exposure is currently outstanding.
Open interest is the raw material from which gamma exposure is computed. Two strikes with identical volume but very different open interest carry very different dealer-positioning weight. ChartGEX weights GEX by open interest, then layers a volume-weighting overlay (VW-GEX) that emphasizes strikes with active flow.
What it does not tell you
Open interest does not tell you the direction of dealer positioning — only the size of it. A 25,000-contract call OI at a strike could be net dealer-short (calls bought by retail and short-dated traders) or net dealer-long (calls sold by yield-seeking funds). Gamma exposure resolves the sign by convention, and dealer-structure detection refines it further using the IV trough signature.