The Nets claimed a second straight win on Sunday, with O'Neale's three-pointer with less than 30 seconds left on the clock nosing them into a two-point lead.
Golden State had been leading 106-93 midway through the final quarter on Sunday, and O'Neale hailed the Nets' attitude and self-belief.
"There's a lot of confidence going around," O'Neale told reporters.
"Everybody's taking the challenge, stepping up, finding out ways to win games."
Irving was key, teeing up O'Neale's crucial three-pointer to cap a ninth assist of the game. He finished with 38 points and seven rebounds.
"Kyrie had been getting to the basket, hitting tough shots and at that moment, he drove, two people double-teamed and he found me open," O'Neale said.
"I just needed to have the confidence to knock it down."
Irving added: "They did a great job of staying on my body, keeping the game physical, making it tough.
"I felt like my team-mates had a lot of great looks, drawing the defense to lose their man.
"Royce being wide open, I felt like that was the best shot for our team, so I gave up the ball, trusted him to make it and luckily it went in."
The Nets are fourth in the Eastern Conference, while the Warriors – reigning NBA champions – are down in 10th in the West after suffering a sixth loss from their last nine games.