Study of the oral examinations with qualitative research methods showed that the differences in perceived competence were most marked when the oral examinations assessed how students thought about problems.
Source: Wright, J. C., Millar, S. B., Kosciuk, S. A., Penberthy, D. L., Williams, P. H., & Wampold, B. E. (January 01, 1998). A Novel Strategy for Assessing the Effects of Curriculum Reform on Student Competence. Journal of Chemical Education, 75, 8, 986.
See also John Bryan Starr's use of oral exams from his episode of the Teach Better Podcast