How to read option chain history
An option chain history is a series of dated snapshots, not a single live quote. Read it snapshot by snapshot, one expiry at a time.
One snapshot is one moment
Each row in a chain history ties one underlying, one expiry, one strike, and one observation date to a call and put open-interest figure. Treat every snapshot as a photograph of that moment, not a continuously updated feed.
Keep volume and open interest apart
Daily volume counts contracts traded that day. Open interest counts contracts still outstanding. A history that blends the two under one column will misrepresent both; keep them as separate series even when a source reports them side by side.
Compare like with like
Line up snapshots by the same underlying and the same expiry before comparing open interest across dates. Comparing across different expiries, or across a contract multiplier change, produces a chart that looks continuous but is not measuring the same thing.
Read the source before the number
Record where each snapshot came from, when it was fetched, and whether the publisher licenses redistribution. A number without a dated source and a stated multiplier is not reproducible, and reproducibility is the point of keeping history at all.