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Open interest of SPY: https://finance.yahoo.com/q/op?s=SPY+Options

If someone sell short a contract the open interest adds up or down? The open interest on yahoo finance is a reliable Information of how many contracts are really opened or they misses someone?

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Options are in zero net supply (like futures and other derivatives), so for evry long there is a short and for every short there is a long. The open interest is the sum of the longs which also equals the sum of the short positions.

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