The difference in usage is not localized English: it's cultural student/academic vs. employed.

To a student with no professional experience, "office hours" are posted times when a professor is available to further mentor and guide a student. "office hours" are designated times outside of the scheduled class times and professors are generally required to provide them so others can benefit.

To a professional and others with white-collar jobs, "office hours" are hours are times when an employee must be seen by a manager to be considered working. Times outside of the scheduled work times are called "outside of office hours" and employees are generally expected to provides them so others can benefit.