I don't think princess Olga was ever born HRH officially. Prince Michael's mariage was considered unequal , and King Constantine approved of the marriage only after prince Michael relinquished his place in the line of the succesion to the Greek crown for him and his descendants ( but he kept his title). So when Olga was born she was not HRH princess Olga of Greece and Denmark but was called just princess Olga of Greece ( since her father was an HRH , it seemed fair to the Royal Family to call her a princess even though it was more of a honorific title in her case than a dynastic one). she only became an HRH AFTER her marriage as HRH,The Duchess of Apulia. So it couldn't just be HRH Princess Olga of Greece in the passport. It could only have been HRH Pincess Olga , The Duchess of Apulia.
What kind of passport did she have? I would find it very strange if it was a Greek one...