Open interest for options is similar to shares outstanding for stocks. Open interest represents the number of open contracts on an option at the end of each day. Each side of an option order must be to open a new position or to close a preexisting position, regardless of which side is the buyer and the seller. If both parties in a transaction are opening new option positions, then that will cause the open interest on that option series to increase. If one side of the trade is to open and the other side is to close, then that trade will have no net effect on open interest.
Open interest is a smoother piece of data than volume.
Volume starts anew each day, so it is quite possible for volume to differ dramatically from one day to the next. On the other hand, each new day's open interest is calculated by starting with the prior day's open interest. It can be seen that the total put and call open interest numbers change much more slowly than their volume counterparts. The open interest ratio ranges from 0.72 to 0.78, while the volume ratio jumps ranges from 0.34 to 1.31. It just means that each of them must be prepared in different ways.