The show will be televised live and exclusive on the Channel of Champions BoxNation, (Sky Ch. 437, Virgin Ch. 546).
O'Sullivan, 28, was in action last July at West Ham United's ground on the David Haye-Dereck Chisora undercard when he defeated Manchester puncher Matthew Hall over twelve rounds to claim the vacant title.
Known as the Celtic Rebel and trained by Paschal Collins - the brother of former WBO World Super-Middleweight Champion Steve, O'Sullivan is unbeaten in 15 fights with 9 stoppages.
"I'm the champion and I'm relishing the chance to go to Liverpool and defend my title against Selkirk in his back yard. I've boxed all over the place in America, Scotland and England, as well as in Ireland. It doesn't phase me or bother me that I'm on his patch, it really appeals to me. I won the title on the road and now I'm defending it on the road. Selkirk's a good tough fighter, he just wants to fight and so do I, the best will rise. I believe I've got the tools to beat him, but it's going to be an exciting fight."
Selkirk is eager to be the next big star to emerge on the thriving Merseyside boxing scene.
The former amateur sensation, who won an ABA title and competed in the Junior Olympics, has put in some eye-catching performance in the pros, so far racking up an unbeaten 10 fight record and stopping 5 early.
Selkirk says challenging for the first title in his career will take his performance to another level.
He said, "I'm really excited about this one, I'm treating it like a world title. O'Sullivan is a good fighter, I'm not going to knock him, he's unbeaten and has got a good record. I'm unbeaten as well and I'm looking forward to testing myself against him. I know how good I am and how good I can become and this is the fight where it will come out. This is the type of fight that I need to get the best out of me and you'll see that."
Ellesmere Port sensation Butler headlines a talent packed show when he challenges Commonwealth Super-Flyweight Champion Kareem for the title. Also on the card is Ellesmere Port's Matthew Fagan; Liverpool's former British Super-Featherweight Champion Stephen Smith; Lincoln light-heavyweight Calum Johnson; Welsh prospects Craig Evans (super-feather) and Liam Williams (Clydach Vale).
Remaining tickets priced at £30, £40 and £60 are available from Eventim on 0844 249 1000 or eventim.co.uk
Watch Butler v Kareem and O'Sullivan v Selkirk live and exclusive on BoxNation, (Sky Ch. 437, Virgin Ch. 546) on Saturday 12th April. To join go to www.boxnation.com
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For just £10 per month BoxNation offers unbeatable value for money for all fight fans. To subscribe to BoxNation simply go to www.boxnation.com and hit the "Subscribe Now" button and choose your subscription package.
BoxNation is the first dedicated subscription boxing TV channel in the UK to bring together the biggest names in amateur, domestic and international boxing with an unbeatable schedule of matches from across the globe.
For just £10 per month, BoxNation offers unbeatable value for money for all fight fans. To subscribe to BoxNation simply go to www.boxnation.com/subscribe and choose your subscription package. Simple!