A second-year student at the University of Bath has sold his internet business for $1.25m.
"With exams, I have not really had time to celebrate yet," said O'Neill.
"The website started off as a way of showing Flash animations I made to my friends and just grew from there. I was learning things in my business studies A-Level course and applying them to my business in the evenings."
Holy Lemon also hosts games, pictures and videos, and boasts 1.1 million unique visitors a month. O'Neill's site started before YouTube, which he first saw as an interesting competitor.
"At the time of its launch I remember seeing YouTube and thinking that it was good. This was back when my site was 10 times the size," said O'Neill.
"YouTube's success came through a widget that allowed people to distribute video content, but they had a team of developers and a lot of money to get it where it is today."
O'Neill is currently working on an online magazine and forum for gamers and is planning to start a social networking site in the UK later in the year.
Siobain Hone, student enterprise co-ordinator in the University of Bath Students' Union, said: "Kieran is testament to the ability of students and young people in general to become entrepreneurs.
"While Kieran is at the top end of entrepreneurial activity, he should be a huge inspiration for the many students who have great ideas for enterprises."
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