| Archbishop Denis O'Connor Catholic High School | |
| Motto: | Nil sine fide et labore |
| Schoolnumber: | 703923 |
| Established: | September, 1964 |
| Campus: | Suburban |
| Grades: | 9-12 |
| Principal: | Mr. J. MacNeil |
| Enrolment: | 585[1] |
| Enrolment As Of: | 2019/2020 |
| Athletics: | Baseball, Soccer and Basketball |
| Colours: | Green and Yellow |
| Free Text: | Green, Grey, and White |
| Free Label: | Uniform colours |
| Address: | 80 Mandrake Street |
| Postalcode: | L1S 5H4 |
| Website: | 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.
Archbishop Denis O'Connor Catholic High School was founded by Father Leo J. Austin and was officially opened in September, 1964 in Whitby, Ontario. The school was moved to its new home of Ajax in 1984. On November 18, 1984, it was officially opened and blessed by Cardinal Carter. In 1991 a major addition was constructed which increased its capacity to approximately 1,000 students.[2]
The school is home to a wide variety of athletic and academic activities. It has highly talented sports teams, such as their senior boys basketball team and Varsity Boys Baseball Team. The senior boys' soccer team has proved to be one of the most successful teams in the Durham Region, winning five Lake Ontario Secondary School Association (LOSSA) championships in a row.
The school's students generally come from the following Catholic Elementary Schools which are all located in Ajax: