In tribute to the 54-year-old principal and his great sense of humour, the service was kept light-hearted and filled with anecdotes of his quick, dry wit.
Deputy principal Steve Read said O'Brien would have told those at the mass, in the sweltering school gymnasium, to be comfortable.
"He would say, 'It's very hot in here, and if you want to take your jacket off, do so'.
"You can also loosen your tie - he was a great fan of that too."
Those paying tribute said he was a highly respected educator and a "beautiful" man who treated pupils and staff like family - not just while they were at the school, but long after they had left as well.
O'Brien's body is now in his home town of Whakatane, awaiting his official tangi tomorrow.
Kavanagh College board of trustees chairman Trevor Thomson said Read would be the school's acting principal until a permanent replacement could be found.
"We are hoping the successful person will be able to start in the third term."