He helped the priest prepare the sacraments and organised the choir.
Once, when the priest got stuck in traffic, the former cleric celebrated the mass at Rotterdam's Church of the Holy Heart, Christ our Redeemer, until the priest arrived.
O'Grady also volunteered at the Missionaries of Charity in Rotterdam.
Among other things, the religious order runs a refuge for vulnerable mothers and their children. To earn money, he worked at a fast-food restaurant in the city centre.
Often, the "grandad figure" was in charge of children's birthday parties at the fast food outlet.
None of O'Grady's new friends in Rotterdam knew he was considered one of the world's most dangerous clerical sex abusers until two weeks ago.
In 2006, the former priest achieved notoriety when he agreed to feature in a documentary discussing his sexual abuse of children.
Deliver Us From Evil was aired on national television in Holland on 14 April. Many of the parishioners from his church, where he was effectively acting as a church deacon, saw it.
O'Grady hadn't made friends since he left Ireland for Holland in August 2008. He kept to himself mostly. One family he did form an attachment with was a couple with a nine-month old baby.
The couple were stunned to learn about their friend's past, and it chilled them to learn the convicted paedophile admitted to assaulting a nine-month old in the US.
O'Grady met the young woman, who has previously lived in Ireland, while they were both volunteering at the Missionaries of Charity.
They immediately had something in common, a bond. She too is religious and told him about the weekly English religious service at the Church of the Holy Heart.
He had been attending a different church in Rotterdam but soon switched.
O'Grady became friends with this woman and her husband, spending time with them at their home. He even attended their daughter's christening.
"My first reaction when I saw the documentary was, 'does he have a twin brother?' I just couldn't believe it was him," says the woman's husband, whose identity the Sunday Tribune is protecting.
"Shock isn't even the word. He was in our house maybe two or three times, having a couple of drinks while I helped him with paperwork. He was trying to get social welfare. We felt sorry for him. He was alone, had no friends or family here. He really was a very nice man. He was sociable, well-spoken and very refined. There was never anything about him that would suggest anything was wrong. He was a grandad type. That's scary now to think of. You would never suspect a thing."
It is every parent's worst nightmare.
A convicted sex offender trying to ingratiate himself into your family.
The couple has taken some comfort from the fact that O'Grady was never alone with their daughter.
But still, that this man was able to easily become a part of their lives troubles them.
"We invited him to our home. He wasn't pushing himself on us. When I saw that he had assaulted a nine-month- old baby, I was speechless. How could someone do that? Was he trying to groom our family? I don't know. What could have happened, I just don't know," he adds.
O'Grady was always vague about his own past, saying he used to work in administration in Ireland and moved to Holland to be closer to his nephews, who he said were studying there.
But he told other friends at their church a different story.
He said he had worked for RTE but had been fired when he had a nervous breakdown.
"At church, he used to organise the singing. He would help the priest with the bread and wine too and make announcements, stuff like that.
One Sunday, when the priest was late, he took over the service. I remember saying to him afterwards, 'you were very good, you should study to be a priest'. Little did I know," his former friend says.
After Deliver Us From Evil was aired, an announcement was made in the church informing the congregation of O'Grady's criminal past.
The 64-year-old had already left the country.
About three weeks ago, his neighbour was walking past when she saw the police knocking on his front door. He was not home.
She asked if she could help in any way but they said no.
O'Grady had in fact returned to Ireland in February, telling people he had family business to attend to, but returned briefly to Rotterdam two weeks ago and was last seen wearing a fake beard at Central Station on 14 April.
It is not known why the Dutch police were looking for Oliver O'Grady.
"I didn't know him very well, he was always very polite," says his neighbour, who asked not to be named.
"I know he used to work at [a fast food] place and he had people from the church around at his apartment sometimes. The woman he got the apartment from is involved at the church. I plan to confront her and ask if she knew about his past. My children are grown up but there are others around here."
After he returned to Ireland, O'Grady emailed his friends – the couple with the nine-month old baby.
He wrote on 30 March: "Glad to hear that all goes well at the church. Great to hear that a choir actually is in operation. I had doubts about it and was at my wits' end trying to think of ways to get things going in that area. One of the big problems was trying to get people to either practise (like before mass) or to pick songs that people knew… Regarding myself, I do apologise that I had to leave very quickly and to come back to Ireland… I'm still trying to get the financial things straightened out here and hope that falls into place real soon. I am, however, getting a supplemental income [a form of social welfare] for the time being, so I'm not too badly off."
O'Grady also enquired about their baby daughter.
"She must be beginning to move about and chat a lot by now… Thanks, once again, for all your help while I was in the Rotterdam area. Hope to be in touch again real soon. All the best, Brother Francis."
The English service at the Church of the Holy Heart does not have one dedicated priest, but a group of rotating clerics who celebrate mass each Sunday.
When one of the priests, Fr Avin Kunnekkadan, was informed about O'Grady's convictions for sexual abuse of children by church members, he said he knew nothing about O'Grady's past.
He wrote in an email to a parishioner: "I too had helped Francis in a small way without knowing his background. He came across to me as a gentleman… It was a big mistake that he has hidden his identity. Maybe he is a changed man. But who knows the truth. Only God can understand his intentions…
"Now I feel sorry that I did not ask any personal background of Francis when I met him first in the church. I took him as a matured gentleman. I never entered into his personal life story… In all sincerity I do not judge the past life of Francis as judgement belongs to God. I assure you that I will speak to the parish council and see what can be done. However, this matter cannot be hidden away from the community."
O'Grady spent his career as a cleric in the US, having been sent abroad after he was ordained.
He has admitted to sexually abusing over 20 children but has only been convicted and jailed for sexually abusing two brothers.
He was deported to Ireland in 2000.
"By calling himself Brother Francis, he's implying he's attached to a religious order. He was acting as a deacon in the church," says John Manly, a partner at law firm Manly, McGuire & Stewart in California.
The firm specialises in representing clerical child- abuse victims and has served civil action against O'Grady.
"I think he has help within the Catholic church. How could he walk into that church in Rotterdam and start helping out? I don't believe for a minute they didn't know who he was."SIC: ST