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?
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.