Open interest builds a pin, but the strength of a pin isn't the raw contract count. Two strikes with identical OI can pin very differently, because what actually holds price is gamma — and gamma depends on more than how many contracts sit at a strike.
What decides pinning strength
Gamma concentration, not contract count. As with the walls, a strike's pinning power is its open interest weighted by gamma. Distance from spot: gamma is highest at-the-money, so an OI pile far from price pins weakly even if it's large. Time to expiration: gamma spikes into expiry, so the same OI pins far harder on expiration afternoon than a week out (the roll-off). A strong pin is near-the-money, near-dated, and gamma-heavy — not just a big number.
A giant OI far from spot is a weak magnet. A modest OI at-the-money on expiration day is a strong one. Gamma, distance, and time — not the headline count.
Reading a strong vs weak pin
Judge a potential pin by proximity and timing, not size alone. Price sitting near a heavy strike late on expiration day is a strong-pin setup — expect stickiness, fade the edges. A big OI strike that's $10 away, or days from expiry, will barely magnetize price and shouldn't anchor your plan. The tape confirms it: a real pin shows price repeatedly drawn back to the level; a weak one lets price wander.
Why it matters
Misjudging pin strength is how traders get chopped — fading the edges of a “pin” that was never strong, or trusting a level that had the OI but not the gamma. A gamma-weighted map shows pinning strength directly, so you're trading the force that actually holds price, not the contract count that merely suggests it.