If person A is 100 contract call long, B is 100 call short, C is 1 call long and D is 1 call short. Now when A & B close their position with each other, the volume will be 100. But since C&D still have open positions, so OI will be 75 (1 contract=75 shares). So volume=100 and OI=75.
SOMEONE CORRECT ME IF NEEDED.