This paper uses a newly discovered data set on examination results by gender from 1913 to 1986 to show that the gender gap in examinations at the University of Oxford has existed for over a century. We show that after declining for almost 70 years the gender gap in Firsts increased significantly after the introduction of coresidence -- the admission of women to formerly men's colleges in 1974. The gap in Thirds between men and women increased between 1953 and 1977 but then declined significantly after the mixing of the colleges. We found that coresidence affected the gender gap in both Arts and Sciences and Combined subjects, not just in sciences as was commonly believed.