Prior to the introduction of OAC for the 1984-1985 school year, Ontario had 13 grades. There were two high school diplomas in Ontario, the Secondary School Graduation Diploma (SSGD) which was awarded after Grade 12 and the Secondary School Honours Graduation Diploma (SSHGD) awarded after Grade 13. The "Grade 13 diploma" was recognized in some jurisdictions as being the equivalent of first-year university and having it would enable some students to apply directly for entry into second-year at universities outside of Ontario. This practice ended with the replacement of the SSHGD with the Ontario Secondary School Diploma (OSSD) under OS:IS.
OS:IS more formally allowed for the completion of schooling after only 12 grades, where previously, this had been an exceptional circumstance. Under OS:IS, OAC year was the final year of high school in Ontario. Students were not required to complete that year in order to receive the OSSD; many students graduated after Grade 12. However, Canadian universities (and indeed, most universities abroad) required OAC for admission.
OAC courses were the highest level courses in Ontario high schools until the formal elimination of the Ontario Academic Credit. To enter university, students were required to complete 30 high school credits (courses can have different credit values, but most courses were worth 1 credit; some courses were compulsory and there were other restrictions), 6 of which had to be at the OAC level. Assuming that one had taken the necessary prerequisite courses, one could complete OAC courses before the OAC year, thus in many schools, it was common for Grade 11 or Grade 12 students to take OAC courses. Students who completed these requirements in 4 years of high school were permitted to graduate; this practice was known as fast-tracking. Finishing Grade 12 in four years with 30 credits was simple if the student was college bound. However most student who were interested in studying at the university level, ended up staying for a 5th year to complete OACs due to the heavy workload, lack of OAC prerequisite credits, or poor pre-planning.
Ontario universities looked at a prospective matriculant's "top-six" (the six OAC courses taken with the highest grades) and averaged them. If one's "top-six" average was above a university's "cut-off" (the lowest average they would be willing to accept for that year), one would be admitted. It should be noted that most university programmes had certain course requirements (e.g., humanities programmes typically required at least OAC English; science programmes typically required, in addition to OAC English, a combination of OAC Algebra & Geometry, OAC Calculus, OAC Biology, OAC Chemistry, and OAC Physics), thus these courses had to be completed and are considered part of the student's "top-six."
Students with an average of 80% or greater over all OAC courses were named Ontario Scholars. Currently, the same applies for people getting an average over 80% in their grade 12 courses.
The phasing out of the OAC year, a part of a series of Secondary School Reforms, was announced by the governing Ontario Progressive Conservative Party soon after their election to office in 1995 under the leadership of Premier Mike Harris. Some suggested motivations for these reforms are to modernize the education system or to save money. The reforms cumulated into a new, standardized curriculum documented in Ontario Secondary Schools, Grades 9 to 12: Program and Diploma Requirements (OSS). The OAC year was replaced with an extra ten days of schooling in each lower grade. Most Ontario universities which had offered three-year Bachelor's programs began to phase those out in favour of four-year Honours degrees.
The elimination of the OAC year produced a "double-cohort" caused by both the last OAC (OS:IS) class and the first Grade 12 (OSS) class graduating in the same year (known as the double-cohort year, 2003). This led to more competitive admission standards at most Ontario universities. Some students under OS:IS who feared that they might not be able to gain admission to the university of their choice as a result of the double cohort decided to fast-track to graduate before 2003; a variation of this is where some students under OSS decided to take an extra year of high school to graduate in 2004 or delayed application to post-secondary institutions. Double-cohort students who chose the latter options in their turn affected those in the year after them. In June 2007, a cascade "double-cohort" effect occurs at universities and the job market, as these double-cohort students who finish undergrad in April compete for graduate spaces in universities or employment in the job market.
Informally, OAC is used to refer to the OS:IS curriculum and Grade 12 is used to refer to the OSS curriculum in discussions of the double-cohort.