The total number of one-side positions outstanding in options or future contracts. In other words, the total number of long positions or the total number of short positions represents the open interest in the corresponding derivative. For example, if an option contract is traded, and neither side is closing an existing position, the open interest increases by one contract.

If one investor is closing an existing position and the other is not, the open interest remains the same. However, if both investors are closing existing positions, the open interest will decrease by one contract.