Open interest (OI)
The total notional value of derivatives positions currently open on a venue.
OI rises when a new long and short open together and falls when both close; a position changing hands leaves it unchanged. It measures leverage in the market, not trading activity. Rising price with rising OI is new money entering; rising price with falling OI is shorts being squeezed out. OI large relative to market cap means liquidations will overwhelm spot depth.