Open Interest is different from trading volume. Trading volume is calculated based on all pairs of long-short transactions happening in the market. On the other hand, open interest only talks about the number of contracts that are outstanding or open at that point of time.
Usually, higher the open interest, more is the liquidity and tradability of the security. Thus high open interest indirectly translates to high trading volume as it signifies that there are many buyers and sellers who are generally trading with the security in the secondary market.
A buys 10 futures contracts of SCB
B sells 10 futures contracts of SCB
C buys 2 futures contracts of SCB
D sells 2 futures contracts of SCB
A sells 1 futures contracts of SCB
D buys 1 futures contracts of SCB
B buys 7 futures contracts of SCB
E sells 7 futures contracts of SCB
(i) On Monday, 10 futures have been sold by B and they have been purchased by B -> number of open futures = 10. Hence Open Interest = 10.
(ii) Similarly, on Tuesday, 2 futures have been sold by D and they have been purchased by C -> number of open futures = 10+2. Hence Open Interest = 12.
(iii) On Wednesday, A offsets his position by 1. Hence Open Interest = 12-1 = 11.
(iv) On Thursday, B buys 7 futures sold by E (new party in the market). B is just substituting E’s role. Hence Open Interest remains at 11.
However, Trading Volume comes to 20 by the end of Thursday as every transaction is counted in trading volume (not jut open contracts).