Prior to the introduction of OAC, Ontario had 13 grades. OAC replaced grade 13, and more formally allowed for the completion of schooling after only 12 grades, where previously this was an exceptional circumstance. To enter university, students were required to complete 30 high school courses, 6 of which must be at the OAC level; students who completed these requirements in 4 years of high school were permitted to graduate. Most students continued to stay in high school the extra year.
As a cost cutting move, the elimination of OAC was announced by the Ontario Progressive Conservative Party soon after their election to office in 1995 and scheduled for eight years later. The extra year was replaced with an extra ten days of schooling at each lower grade. Also most Ontario universities which had offered three year Bachelor programs moved to require a four year bachelor program.
The elimination of the OAC year in 2003 produced a 'double cohort' caused by both the last OAC class and an entire class of Grade Twelves graduating in the same year. This has lead to more stringent admission standards and overcrowding at Ontario universities.