What is Open Interest (OI)?
Open interest (OI) is the total number of derivative contracts, such as options or futures, that are open at a point in time and have not yet been squared off, exercised or expired. OI rises when a new buyer and a new seller open a position and falls when both close.
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OI vs volume
Volume counts every contract traded during the day, including positions opened and closed intraday. OI counts only the positions still open, so it shows where money is committed rather than how busy trading was.
How is OI used in an option chain?
Strikes with very high call OI often act as resistance and strikes with very high put OI often act as support, because option writers defend those levels.
In the AOC option chain the highest call OI is highlighted red, the highest put OI green, and the second highest on each side yellow, with the percentage of the maximum shown in each cell.
