The 20-year-old, who can play as a striker or an attacking midfielder, put pen to paper after a successful trial period.

"Sean impressed us during a trial and we think he has a lot of potential," explained Ian Burchnall.

"He's one we can develop over time with a view to cementing him as a first-team player.

"He's a clever footballer who's very comfortable on the ball and has a lot of positive attributes."

O'Brien added: "I'm really happy to be here. It's a great opportunity to further my career and this seems like the perfect club for me.

"I spoke to the gaffer and I've loved being at the club during a trial period. I've enjoyed every day and I can't wait to get started properly now."

Sean will wear number 20.