Volume measures activity, open-interest measures inventory. A series can trade 20,000 contracts and end the day with unchanged open interest if it was all day traders opening and closing, or all closing of existing positions.
Comparing them is the useful move. Volume far above open interest means new positioning is happening now, which is the basic filter behind most unusual-options-activity screens.
Example: the XYZ 20 Mar $55 call shows 300 open interest and trades 9,400 contracts today. Someone is building something. Tomorrow's open interest print tells you whether it stuck: 9,000-plus means new positions, unchanged means it was intraday churn.
Related: open-interest, volume-open-interest-ratio