Why does open interest not increase by 100 if 100 contracts were opened?

Have you ever seen someone ‘open’ 1,000 contracts but the next session the open interest didn't increase by 1,000?

Similarly, have you seen a trader close a 10,000 contract position but the next session the open interest barely budged?

Open interest, as it pertains to an options contract, is defined as the total number of outstanding contracts that exist in any given session. Nothing more, nothing less.

Assume there is zero open interest on a contract. You purchase ONE contract. The next session the open interest increases from zero to one as you'd expect.

Let's say the contract's bid ask spread is 1.00 x 1.20.

I have an open BUY order for this contract at 1.00. If you happened to put in a SELL order for your contract at 1.00 it would be executed immediately with me as the buyer. Assuming no other activity took place in the session the volume for today would be one. The open interest the next session would still be one because that is the total number of outstanding contracts.

Do you see how YOU closed one contract, but since I opened one there is a net change in OI of zero?

Let's scale this example up slightly.

Hope these examples help!