Oldaker has joined from the Shrimpers from Swindon Town, signing a two year deal.
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And the midfielder is pleased to be working with Blues head coach Dyer once again after their successful spell together at Chesterfield.
"Him being here is a big reason why I joined," said Oldaker.
"He looked after me at Chesterfield and made me a better player."
“I’m delighted to be here,.
"This is a big club with huge ambitions and I want to be a part of it.”
“I’ve got a lot of faith in the manager and the way he wants to play football."
And Dyer is also pleased to have the 27-year-old on board.
“I mentioned that I want to dominate the ball, and he’ll be key to that," said Dyer.
“He’s very brave, has outstanding ability, and can pass with both feet.
"He’s got a wide variety of passing range, hitting long balls and breaking lines.
"He’s one the fans will definitely enjoy.
"Technically he's one of the best I've worked with and he will be a big part of our future."
Oldaker has previously played for Gillingham, Billericay Town, Welling, Dorking Wanderers, Chesterfield and League Two side Swindon Town who he played 45 times for last season.
Oldaker worked with Shrimpers boss Dyer during their time together with Chesterfield which saw the Spireites crowned National League champions for the 2023/24 season.