Why a daily count falls behind
OCC publishes open interest once a day, reflecting positions at the previous close. During the session, new trades open and close positions that the count does not yet show. Any exposure built from open interest is a morning snapshot that the day's trading gradually changes.
Volume and open interest measure different things. Volume counts every contract traded today, whether it opened a position, closed one, or moved it between holders. Open interest counts only what is still open.
Reading a trade
To guess whether a trade was a buy or a sell, analysts compare its price with the quotes. A trade at the ask was probably buyer-initiated; at the bid, seller-initiated. For options this quote rule is right about 83% of the time (Savickas & Wilson, 2003). Signed option volume has been found to carry information about future stock prices (Easley, O'Hara & Srinivas, 1998).
Example: 5,000 calls trade at the ask, five times the contract's normal daily volume. That looks like aggressive buying. It could be a new bullish bet, a short seller covering a position, or one leg of a spread whose other leg traded elsewhere. The next morning's open interest shows whether open interest rose in total, but not which trade did it.
Unusual is not the same as informed
A large or aggressive trade is worth a look. Its size and urgency do not tell you whether the trader knows something, is hedging, or is closing a position.
Caveat: data that labels the type of participant and whether a trade opened or closed a position does exist, for example Cboe's Open-Close data, but it covers specific exchanges and arrives with a delay. Treat any intraday update of exposure as an estimate to check against the next official count.