The former Millwall forward became Phil Parkinson’s first summer signing after putting pen to paper on a two-year deal on Thursday.
The 26-year-old has told fans he ‘lives for’ scoring goals and hopes that a successful campaign can lead to promotion back to the Championship and more caps for the Republic of Ireland.
“I was at home when my agent said that Phil Parkinson was interested and wanted to give me a call,” O’Brien told safc.com.
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“I went to the training ground and met him, and to see the place.
“As soon as I got there, I just knew it was the place for me.
“I'm so happy to be here.”
O’Brien also spoke to former team-mate Conor McLaughlin ahead of the move.
“The decision was already made for me but I was obviously at Millwall with Conor McLaughlin,” he said.
“He's a good friend of mine and he was telling me the ins and outs, what a great bunch of lads it is here.
“It was a no-brainer, really.
“I love scoring goals, that's what I live for,” O’Brien added.
“Hopefully I can bring that to the team.
“I know getting out of this division is not just about talent, it's about having a good bunch, a good camaraderie, a good dressing room.
“It looks like we've got that here, and if you have that then the talent takes care of itself.”
O’Brien currently has four full international caps, with one goal.
It’s a tally he hopes to add to as he pushes for a regular slot in Parkinson’s XI.
“That's what I'm aiming to do,” he said.
“Score goals, hopefully get promoted and hopefully get back to the Ireland set-up.
“That's my dream.”
Speaking on Thursday, Parkinson said O’Brien’s versatility would be key in the upcoming campaign.
"He’s at a good age, he has experience of winning this division and he has played plenty of Championship football,” he said.
"Aiden can also play in a variety of attacking roles, which I feel is important for us.”