Archbishop Denis O'Connor Catholic High School

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 who was born in Pickering in 1841, and who became the third Archbishop of Toronto, Ontario, Canada in 1899. The school has students in grades 9–12 and offers a wide range of academic and extracurricular activities. The current enrolment is approximately 600 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."

In 2024, the school celebrated the 60th anniversary of its opening in 1964.