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.