Hi Yuquan,
In past experiments I too have found it to add the inverse of the offset DAC. I found it easier to think in terms of the desired end result. If you have a positive offset at the input, the desire is to subtract the value of the offset DAC to get a 0 result. If you subtract a positive value from a 0 offset you get a negative value.
Personally I have never found a good enough reason to use the ODAC. I think a SYSOCAL is a much better approach for adjusting offset. If attempting to increase the dynamic range it may be helpful, but it also can add error and drift. Also the additional noise can negate the benefit of increased dynamic range.
Best regards,
Bob B