The 18-year-old stormed to victory in the women's 200m individual medley ahead of Australia's Alicia Coutts and Scotland's Hannah Miley, after previously winning three silver medals and one bronze at the Games.
She told BBC Sport: "I'm absolutely over the moon. I knew it would be so hard to get a medal because of the strength of the field so I can't ask for any more.
"Hannah's been my role model and it's been great to train with her."
O'Connor broke the Games record in the process of winning her gold medal.