O'Neill was first opened in 1909 as Oshawa High School on its present site. In 1924, the school was expanded and came to include many distinctive features. These included an auditorium, which was capable of seating 1200 people, with surrounding balconies allowing access for another 300. This area was also equipped with a stage and a glass ceiling to let in natural light. This was considered the social hub of the school at that time. Also added in that year was a gymnasium reaching the full height of the school. It featured a glass ceiling and an indoor track. This gym was large enough for a basketball court and included a viewing gallery.
The school was further expanded and reopened on May 7th, 1930 as Oshawa Collegiate and Vocational Institute. It now featured O'Neill's distinctive front arches and terrace though the area was only used on special occasions. At this time, girls entered the school from the north side and boys entered from the south. The girls gym had been added and is still located as the activity room today. This also marked O.C.V.I.'s becoming a Vocational Institute. A wing wrapping around the original outline of the building was added and contained the vocational rooms for subjects such as Household Science, Home Nursing, Woodworking and Mechanics. The cafeteria was also added at this time, and though small, holding only 160 people, remains, albeit expanded in its current site. At this time, O.C.V.I. also had a large lawn for sports, in the area the parking lot now occupies.
It was 30 years later that the changes transforming Oshawa Collegiate and Vocational Collegiate into O'Neill C.V.I. would take place. In 1960, corresponding to a second high school, being opened, O.C.V.I. was renamed. It was named in honour of A. E. O'Neill, the "father" of O.C.V.I., having served as its principal for 25 years. The 60's also marked physical changes to O'Neill as the former library was divided into two classrooms, one of which is the Social Science office today. The principal's office was expanded into a nearby classroom to occupy it's current position. Another classroom was sacrificed to create the guidance offices, now an important part of O.C.V.I. life. Some of the ground floor shops were removed and became classrooms for other subjects, leaving only the wood shop and metal shop standing today. It was also during this time period that the current gym was created. The boy's gym was no longer necessary and was divided into four classrooms. The arched entrance to the original gym can be seen from the girls foyer, now ushering students into the health classrooms. The upper floors of the gymnasium gave way to more floor space, as the second floor space is now the music room and the third floor became a study hall. That area has also been divided into more classrooms. The creation of a new gym coincided with the addition of the commercial wing, now housing computer labs, Art and Business classes. The current library was added at this time as well, as part of the Business wing attachment. Then came a sadder change. The auditorium was reworked. The balconies were bricked over so there was no longer direct access from the halls and the balconies were removed. The glass ceiling was also covered over to fully enclose the area. It was at this time that the elevated seating was added and a new stage was installed, creating the auditorium visible today.
O'Neill has undergone many changes throughout its history, but remains a landmark at 301 Simcoe Street North, on the same location Oshawa High School was opened many years ago. And, as a final note, there was never a pool; it was included in the original plans, but due to cost, was never actually built.