Open Interest is the total number of outstanding contracts that are held by market participants at the end of the day. It is the total number of futures contracts or option contracts that have not yet been squared off, expired or fulfilled by delivery. It is used to confirm trends and trend reversals and measures the flow of money into the market. For each seller of a futures contract there must be a buyer of that contract. Therefore, to determine the total open interest for any given market you need to know only the total from one side.