Former members of the OUSA executive team came from far and wide for the occasion, including former Court of Appeal and High Court judge Sir Bruce Robertson, and Dame Elizabeth Hanan.
During the day, pre-booked tours of the entire campus were also leaving from the Clocktower.
Dunedin woman and former TV producer Lorraine Isaac, who attended the morning tea, said when she was at the University of Otago in the 1960s there were only about 3500 students enrolled.
Today there were about six times as many. In her day, 80% of them had been men and there were only two women on the executive of OUSA.
Among the events she got involved in were the "big sleep in" in the university common room, in protest against the rules forbidding mixed flatting.
Dr Dennis Pezaro, former OUSA president and former head of the New Zealand Medical Association, said he studied there from 1962 to 1968, and the union building was originally a place where students mingled with staff.
"It's wonderful, it's a special place, this building."
There were "a lot of old friendships" established there.
OUSA, of which he was a former president, was a "very constructive outfit" when it came to getting facilities for the university, and taking over the University Bookshop.
"I think the university is great, " he said.
"I think we are all very proud of the institution we contributed to."
Ex-OUSA executive members came from around the country, as well as overseas for the occasion.