The extension has added another three classrooms to the Loughgall school.
In the course of the visit, the Cardinal toured the school and met the pupils, invited guests and parents, including former principals and teachers.
During the official blessing and opening ceremony, the Cardinal was assisted by the Rev Fr John Connolly, chairman of the Board of Governors.
The Cardinal praised all who were involved in the planning and construction of the new building, including the architect Henry Daly and the contractor Seamus Rice.
He also had words of praise for the school choir, under the directorship of teacher Mrs McParland.
Following the opening, there was a reception in the assembly hall, organised by the Friends of St Oliver Plunkett's group, which gave the Cardinal a chance to have a chat with everyone in an informal setting.
The hall had also been decorated with old school photos which proved an enjoyable trip down memory lane for many.
Principal of St Oliver Plunkett's Mr Jimmy McKee said, "It really was a day to remember.The school looked resplendent with the many wall displays in the classrooms, corridors and assembly hall.
"It was wonderful to see former principals Pat O' Hanlon, Martin Donnelly and Margaret Heaney and former teachers Mary Mc Afee and Marie Mc Cooey mix and chat freely to former pupils.
"The late Master Joe Rafferty was represented by his son Phelim Rafferty and the late Mary Digby was represented by her sister Tessa Digby. This spoke so highly of the quality of relationships fostered and respect observed."