I figured this one out
Its simple maths, volume is additive only whereas Change in OI is subtractive for OI that close.
so in the morning, say OI was increasing along with volume.
Now both have say, COI 10k and VOL 10k.
Now as positions close, OI starts dropping and can approach 0.
After that COI will start registering Negative OI values.
This means OI which was opened on previous days is also closing positions.
Anyway, OI starts from series start and will start from 0 and go +ve (no reset)
Volume will be 0 at day open and go only +ve (resets to 0 at each day open)
COI can have a range of -ve to +ve (resets to 0 at each day open)
So COI can be 0, same time its volume will be 100k.