The final auditors report has a lot of comments known as Opportunity of improvement (OFI) some are makes since and some are not to us. One of my advisers told me that, if I didn't do anything regarding these OFI the auditor will raise theme as minor NC in the next visit, is that true and I have to do something?
It is not true. The Opportunity of Improvement are recommendations of the auditor, and never will be a Non-Conformity. However an observation yes, this can become a Non-Conformity if the organization do not resolve it.
For the internal audit the situation is the same, but in this case is would be interesting that you include in your procedure of internal audit the definition of each finding (Non-Conformity Major, Non-Conformity Minor, Observation, Opportunity of Improvement).