Pene Pati has already spent a year in Cardiff under O'Neill's tutelage, refining a voice which has been likened by Dame Malvina Major to the world's most commercially successful tenor, Luciano Pavarotti.
The threesome were supposed to begin classes this week but Pene requested a late start so the group could tour to raise the $120,000 they need for fees and visas.
"We're up to about $80,000 now and that's purely done through the concerts."
What the brothers, from humble beginnings in Mangere, did not expect was a sellout success.
At the Auckland concert on Tuesday, where the trio went from classics to contemporary music playing the ukulele and bongos, wealthy businessman Owen Glenn told Pene he "absolutely loved it".
"He said he was going to fly our parents to Wales for Christmas. I cleaned out my ears and said 'What did you say?'."
Mr Glenn had already given the brothers, Mr Mackay and soprano Amina Edris $10,000 each to attend the school in Wales.
Pene, who majored in voice performance at Auckland University, has also secured a $30,000 scholarship and a coveted place at the 2013 Georg Solti Accademia summer school in Italy valued at $10,000, for winning the Bel Canto Award.
The former hip-hop and R&B music producer said he went to his first opera in 2008 and decided then to switch genres.
He and Amitai, who also studied voice performance at Auckland University and has toured with the New Zealand Youth Choir, learned to sing at home as children.
With their father Pene snr, mother Juliette and two older sisters all capable singers, the boys grew up with singing and music as second nature.
"My dad was a manager of a rest home and we sang for the elderly every Friday."
Dame Malvina Major said the Pati brothers were a "phenomenal sound".
"Pene is remarkably like a young Pavarotti that I first heard in England. But I just think it's phenomenal to have two tenors of such quality from one family."
She said the situation was unique and the pair had such talent it could lead them anywhere they chose.
"They have the potential to do whatever they like."