It's stated that open interest is updated at end of day after OCC clears all option traded and done with the match up of open and close orders. and I am fully aware of the relationship (or lack of) between volume and open interest. Still there're many cases where daily volume and open interest just don't make sense. Let's look at one example: RAX dec 50call, date, open interest, volume 7/11, 2852, 2055 7/12, 4721, 3 <--- make perfect sense, assuming 2055 contracts are mostly 'open' and not matched by 'close'. However, 7/13, 2724, 10 <--- Now this really needs some explanation! How can the OI be reduced from 4721 to 2724 by only of 3 contracted on 7/12? Recall the stock price and dec 50 C on 7/12 is $42.x, and $2.x respectively so there's NO WAY the holder of the > 2000 call contracts will exercise the OTM options. I've seen this type of behavior many times and asked thinkorswim (supposedly the option trading expert) but never got any explanation. So is the OI updated exactly at end of each day? or there're potential dark pool in option trading where the volume is not reflected?