Open interest
Also called OI, put/call open interest
Open interest is the number of option contracts currently outstanding at a strike — positions opened and not yet closed, exercised or expired.
In more detail
It differs from volume. Volume counts contracts traded during a session, including trades that close existing positions; open interest counts positions that still exist. A day of heavy volume can leave open interest unchanged if it was all closing trades.
Open interest updates once per day after the close, so it describes standing positioning rather than what is happening right now. That makes it the right input for position-based gamma and the wrong input for reading intraday flow.
The split between call and put open interest, often expressed as a put/call ratio, is a rough read on how positioning is distributed.
How Hermes measures it
Hermes publishes total, call and put open interest per ticker, and uses open interest as the basis for position GEX, distinct from the flow-based GEX it computes for SPX.
Read this on a live chart.
Hermes puts dealer positioning next to price through the session. Free to use.