Free Presbyterians held a protest last Sunday as a Catholic priest was installed for the first time in St Anne's Cathedral, Belfast.
Originally from Magherafelt, he has been the parish priest of St Brigid's, south Belfast, since 2010, and continues to hold the post.
The 65-year-old has been installed as an honorary Ecumenical Canon.
Free Presbyterians waved placards and read Bible verses outside as the ceremony went ahead.
Fr O'Donnell, who was educated at St Malachy's College in Belfast, will join a Methodist and Presbyterian minister - both, by coincidence, called Rev Ruth Patterson - on the Cathedral Chapter.
In July, Fr O'Donnell, who recently celebrated his 40th anniversary as a priest, told the Belfast Telegraph that he acknowledged that the move will be a challenge for some Protestants and Catholics.
He said he recognised it was "bound to cause ripples on both sides".
Catholic canons have already been appointed by the Church of Ireland in Armagh and Dublin but the move is a first for Belfast, where close relationships have already been forged down the years between the cathedral and a number of Catholic churches, notably St Peter's Cathedral in west Belfast.
"I recognise that for St Anne's Cathedral community, and for the Catholic community of Belfast, that this is a significant step, perhaps even historic, but more so, I recognise the generosity of the Dean and Chapter in inviting me, as a representative of the Roman Catholic Church, to be an Ecumenical Canon," he said at the time of his election.
"I look forward very much to working with them and making a contribution to the Christian life of our city.
"Our common faith Jesus Christ commits us to strive for reconciliation and unity - that unity, wonderfully described by the great Lutheran theologian, Oscar Cullmann, as 'unity in reconciled diversity.'