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When viewing an option within an option chain browser/UI, why is it that open interest only updates day-by-day, yet volume is updated minute-by-minute?

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Traders submit exercise notices during as well as after market hours and their brokers notify the OOC. In the evening, the OCC uses an electronic "Wheel" to randomly determine broker assignments which are then sent to those brokers who then use their own in house method to determine who gets assigned.

That means that you'll receive assignment notification very late the same day but more likely early the next morning, depending on how soon your broker updates and notifies you. Since this process occurs after hours, open interest is not updated until the assignments are determined and therefore it lags by a day.

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  • But do you need the assignments to calc OI? I would think the exchange could distinguish open/close orders and calc OI regardless of when the assignment is made. But maybe I'm missing something.
    – D Stanley
    Mar 22 at 18:58
  • The exchange has no idea if it is an opening or a closing trade (that's brokerage info). For that to occur, the broker would have to add that information to the order which would require additional technology. Let's carry it a step further. Let's say that I own 10 calls and I place an order to sell 15 calls (10 are STC and 5 are STO). Now the exchange has to keep track of more data, especially if there are partial fills. Then there's the question of how many counterparties are on the other side of my order. One? Five? Fifteen? Suppose some of them are BTO and BTC? It gets kind of messy. Mar 22 at 19:44

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