The cardinal was forced to leave his role as Archbishop of St Andrews and Edinburgh, the UK's highest ranking Roman Catholic, in February after allegations of sexual misconduct were made against him - which he later admitted.
Recent reports have suggested that the Vatican had ordered Mr O'Brien not to follow through with his plan to retire to Dunbar, and instead leave Scotland and live elsewhere.
On Wednesday, the Vatican said in a statement that Mr O'Brien would be leaving Scotland for "several months" with his future to be decided by the Holy See.
But Canon John Creanor, priest at Our Lady of the Waves Church in Dunbar and Our Lady of the Star Church in North Berwick, said that Mr O'Brien, 75, would still receive a warm welcome from parishioners should he choose to retire to Dunbar.
And he has vowed to fight any moves to stop the cardinal moving into a property on the town's Bayswell Road owned by Mr Creanor.
Mr Creanor told the Courier: "I made a commitment to the cardinal that if he wants to retire to Dunbar in time the place will be ready for him, and I am prepared with a legal team to take on the Vatican. He has a right to retire wherever he wishes."
Mr O'Brien has already moved personal belongings to the house in Dunbar having vacated his official residence in Edinburgh, but has not yet moved into the property.
Mr Creanor confirmed that discussions were ongoing within the highest level of the Catholic Church over whether Mr O'Brien should continue with his plan to settle in the county.
He also revealed that parishioners in Dunbar had shown their support for the cardinal's plan to retire to Dunbar, and that personally he was "committed to my friend".
"The people of the parish have made it clear they would welcome him [to Dunbar], that has never changed," Mr Creanor added.
"I am going to do everything I can to see that that happens.
"The parishioners wanted to make it clear to him that they support his coming and wrote their names [on a paper] at the back of the church, which I have sent to [Mr O'Brien] and which he greatly appreciated."
The Vatican's statement said: "His Eminence Cardinal Keith Patrick O'Brien, Archbishop Emeritus of St Andrews and Edinburgh, for the same reasons he decided not to participate in the last Conclave, and in agreement with the Holy Father, will be leaving Scotland for several months for the purpose of spiritual renewal, prayer and penance.
"Any decision regarding future arrangements for His Eminence shall be agreed with the Holy See."