In one line
Volume counts how many contracts changed hands today; open interest counts how many contracts are still alive at the end of it.
How it works
Every option contract has a holder and a writer. Open interest is the number of such pairs outstanding — it rises when a new holder and a new writer create a contract between them, falls when both sides close, and does not move at all when an existing position simply passes from one holder to another. Volume counts every trade regardless.
That is why the CHANGE in open interest, read together with the change in price, is the classification a trader actually uses. Price up with open interest up means new positions being opened in the direction of the move. Price up with open interest down means existing positions being closed. Price down with open interest up is the mirror, and price down with open interest down the other mirror. This desk names those four states and prints one per strike; it prints nothing at all where either input is missing, because absent is not a fifth state.
Open interest is published once a day for the settled session, in the exchange's own files. The intraday picture that data vendors distribute is a different series with different properties, and this desk's daily numbers come from the files rather than from a feed.
What it is not
A large open interest at a strike is not a barrier. That is a specific claim and this desk tested it: on a decade of daily data the heaviest strikes did not act as walls, and near an all-time high the effect ran the other way. What open interest is good for is describing where positions sit, which is information; what it was not able to do here is say what price does when it arrives there.