Open interest (OI) is the number of option contracts currently open (not yet closed or expired). Unlike volume, OI tells you how many positions are live at each strike — i.e. where conviction sits.
OI vs change in OI
Total OI shows the big walls; change in OI shows today's fresh money. Rising price + rising call OI = call writing (resistance building). Rising price + falling put OI = put unwinding (support weakening). Always read them together with price.
PCR (put-call ratio)
PCR = total put OI ÷ total call OI. A very high PCR (lots of puts) is often read as oversold/supportive; a very low PCR as overbought. It's a sentiment gauge, not a signal on its own.
Max pain
Max pain is the strike at which the total value of options expiring worthless is greatest — i.e. where most option buyers lose. Because writers (who are often better capitalised) defend their positions, price frequently drifts toward max pain as expiry nears.