The 24-year-old came through the ranks at Newcastle, who he featured for in the Premier League, before having spells at Carlisle and Hibs.
O'Brien is now looking to get his career back on track with the Robins, and told the club's official website: "I can't wait to get started.
"Hopefully I can create goals and score a few too. I want to help Swindon do really well next season and there will be some great games along the way with the likes of Charlton and Southampton in the division. Most games are big games and I'm really looking forward to getting started.
"I can play out wide on either side and like getting past players. It's about the link-up play too, so creating chances for the team is what I do.
"The club has a lot of ambition and having spoken with the gaffer, he made it clear what he is after and where the club is going. That had a big part to play in the move."