Every strike on a chain displays a bid and an ask, so every strike looks tradeable. In practice liquidity concentrates in a small part of the board and thins out sharply away from it.
Where it concentrates
Near the money, because that is where most interest sits. On round strikes — the psychologically obvious levels attract positioning far out of proportion to anything fundamental. And on the larger expiries, the Fridays and quarter ends from the expiry calendar, rather than spread evenly across every daily contract.
So the tradeable book is a cross: a band of strikes around spot, on a handful of expiries. Away from that cross the quotes exist and the size behind them does not.
What thin strikes do to a structure
Multi-leg positions are the ones that suffer. A spread priced from mid quotes on two liquid strikes may be genuinely available; the same spread using one liquid and one thin strike may cost several times its theoretical price to enter, and considerably more to exit in a hurry.
The exit is the part that gets underestimated, exactly as it is on-chain: entering is voluntary and can wait for a good fill, while exiting is often not. It is the same argument as sizing to the depth you exit into, applied to an options book.
What it does to a dealer map
Open interest at an illiquid strike still enters the gamma calculation at full weight, even if it has not traded in weeks — the stale-strike problem from reading a thin options book.
So an apparent wall on a far strike may be old positioning nobody has bothered to close rather than a live view. Checking whether a strike has recent volume, not just open interest, is the cheap way to tell the difference.
The practical habit
Read the size at the quote, not the quote. And for anything multi-leg, price it at the prices you would actually be filled at rather than at mids — the honest version of the round-trip discipline that applies to every market in this series.