The newly-promoted Championship club have confirmed the 20-year-old's arrival, on a free transfer with a sell-on clause for the Whites, this afternon. He has signed a three-year deal with an option to extend.
Casey first joined his home city club at the age of 10, signed by Andy Wood for the academy having first played for Wigton Moors Juniors from the age of six and then Leeds City Boys.
He has paid tribute to everyone who has helped him progress at Thorp Arch in a message on social media.
"I would like to thank everyone at Leeds United for the past 10 years at the club, staff, players and fans," he said.
"I've gone on to achieve things I never thought would be possible at this club and I couldn't have done it without everyone involved. I wish you all the best in future."
He signed his first professional contract at Elland Road at the age of 18 and 13 months later made his senior debut in a 2-0 Championship win at Huddersfield Town, coming off the bench for the last five minutes. Marcelo Bielsa included him among his substitutes on 13 other occasions in the title-winning season.
Casey's form for Carlos Corberan's Under 23 side in the Professional Development League brought a fresh three-year contract in July 2020 and last season he played 45 minutes in both the FA Cup defeat at Crawley and the EFL Cup loss at home to Hull City. He was also part of the young sides who featured in the EFL Trophy against Barrow and his new employers.
For the 23s under Mark Jackson he missed just three of the Premier League 2 fixtures en route to the league title, forming a solid partnership with Charlie Cresswell at the heart of the defence.
But last summer's additions of Robin Koch and Diego Llorente, along with Pascal Struijk's breakthrough into the first team, meant there was no clear pathway for Casey to progress into senior football with Leeds, so he was keen to get a move elsewhere.
“I’m really excited to get started. Once I knew of Blackpool’s interest, I was really keen to get this done," he said.
“I’ve been at Leeds for 10 years, but I feel now is the right time to move on and make the next step in my career. Leeds have given me a good platform to build on, and what better place to join them from than Blackpool.
“This is such a big club, and to play in the Championship with Blackpool is something I’m really looking forward to.”
Blackpool boss Neil Critchley has a history of working with young players, having previously worked at Liverpool, and believes Casey will be a good fit.
“Olly is a defender with a very good pedigree and is someone we feel will bring real competition to the squad," he said.
“At just 20 years of age, he is a player who has his best years ahead of him. He has had a great education and upbringing at Leeds United, and is someone who we feel will fit in well within the environment we are building here.
“We are looking forward to working with him and helping him develop his game further over the coming years."
Leeds have also received significant interest in Alfie McCalmont from clubs hoping to sign the Northern Irish international midfielder on loan next season, and a number of clubs have also made enquiries over Under 23s player Niall Huggins.