™One trader opens a new long contract against a newly opened short and open interest rises by one contract; this is different from counting both sides as two contracts.
Options
Open interest
The number of outstanding derivative contracts that have not been closed, exercised or expired.
Plain-English context
Put the definition to work.
Count one matched long/short pair as one open contract. Open interest is neither volume nor a directional vote.
Further reading and verification. Examples are hypothetical, not current quotes or recommendations. Product terms, tax treatment and market rules can change. OCC / Options Industry Council — options basics