In the old school photo, Dan is all smiles as poses next to a friend. With strands of his hair sweeping across his forehead, Dan is also flashing his chest in a loose-fitting grey cardigan.
At the tender age of 17, we can't help but think that Dan would have looked perfect in a boyband! We're thinking A1 or Boyzone or perhaps?!
Dan's manager Neil shared the photo on Twitter, saying: "Big Happy Birthday to @DannyO @OnlyWayIsEssex! He hasn't aged one bit from when he was 17! #LovingTheBaggyJumper!"
Dad-of-one Dan replied: "Haha thank you mate. Appreciate you posting that picture too!! Ha!!"
Dan is set to celebrate his birthday in style tonight, as he's holding a big birthday bash at Stadia Norwich nightclub in Norfolk. We wonder if Dan's girlfriend, EastEnders star Jacqueline Jossa, will be joining him too?
Reality star Dan was spotted with Jacqueline on Thursday afternoon (26 June) and kept a protective arm around her as they strolled through Borehamwood, Hertfordshire, and they really do make a gorge couple.
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