Quick definition
The total number of outstanding option contracts at a given strike and expiration that have not been closed, exercised, or expired. Open interest measures established participation, not daily activity.
OI vs Volume
Volume tells you how much trading happened today; OI tells you how much is still outstanding. A strike can have massive intraday volume that closes out flat and adds nothing to OI. A strike can have modest volume that all opens new positions and grows OI significantly. Both matter — for entry liquidity, watch volume; for exit liquidity days later, watch OI.
Why thin OI hurts
Contracts with low OI tend to have wider bid-ask spreads and worse fills. A retail trader who opens a mid-price fill on a low-OI strike may find themselves paying the full spread to exit later, quietly erasing edge. Institutional-grade strike selection weights OI heavily even when the theoretical setup looks best on a thin line.
How Treeova uses it
Arch-AGI's strike-selection layer filters candidate contracts against OI and volume thresholds before an agent is even shown the setup. Kronos, the market-intelligence surface, also tracks large OI shifts as a proxy for positioning by informed participants — sudden OI builds at specific strikes carry signal, especially into events.