Archbishop Denis O'Connor Catholic High School (abbreviated DO'C) is a Roman Catholic high school located in Ajax, Ontario, Canada, within the Durham Catholic District School Board. It is named for Archbishop Denis T. O'Connor, a priest born in Pickering in 1841 who became the third Archbishop of Toronto, Canada in 1899. The school has students in grades 9–12 and offers a wide range of academic and extracurricular activities. The current enrollment is approximately 800-900 students. The school colours are green and yellow, and its motto is Nil sine fide et labore, which is Latin for "Nothing without faith and work."