Michael Appleton has left his job as Oxford boss to take up the assistant managerial role at Leicester, the clubs have announced.
The 41-year-old joined the U's in July 2014, leading them to promotion from League Two in the 2015/16 season as well as two Football League Trophy finals.
Appleton, who has signed a three-year deal at Leicester, will team up with Craig Shakespeare, who was also recently handed a three-year contract after keeping the Foxes in the Premier League.
Appleton has also had spells in charge of Portsmouth and Blackburn, and knows Shakespeare well, having previously worked with him at West Brom's Academy.
And the Foxes manager, who took over from Claudio Ranieri in February with the then champions in danger of dropping down a tier, is looking forward to renewing their partnership.
"I am delighted that the club have been able to bring a coach of Michael's standing to Leicester City," he told www.lcfc.co.uk.
"Michael and I worked together at West Bromwich Albion and it quickly became clear that he possessed all the qualities and the integrity required to enjoy a successful coaching career.
"During his transition from a player to a coach, before later becoming a manager, Michael has already achieved great success in the game. Over the course of his three years at Oxford United, he has been acknowledged by many as one of the most talented and knowledgeable coaches in English football."
Appleton, meanwhile, admits the lure of working in the Premier League was something he could not ignore.
He said on Oxford's website: "I would like to place on record my appreciation for all the support I have had from the board, staff, players and fans of Oxford United during my three years here.
"I have been very happy at Oxford, and would squash any ill-informed speculation that my move has anything to do with the playing budget I'd been allocated, or any uncertainty around the club's ownership. This could not be further from the truth.
"The opportunity to move to a Premier League club was something I felt that I couldn't turn down, which I hope fans will appreciate. I wish everyone at Oxford the best of luck for the future, and I feel sure that the foundations are there for the club to achieve more success in the coming years."
Oxford will now begin the search for a new manager and chairman Darryl Eales has moved to reassure fans the club remain as ambitious as ever.
"We are all disappointed to see Michael go, but I fully understand that the chance to work in the Premier League was an opportunity he felt he couldn't turn down," Eales said.
"Our priority now is to appoint a new manager who will continue our excellent progression and will build on our strong platform which will lead us towards our promotion to the Championship.
"The timing is not exactly perfect, as it does hamper our recruitment plans in the short term but, once the new manager is appointed, we will move quickly to strengthen the squad even further.
"The board would like to thank fans for their patience and understanding and for the countless messages of support. The future remains bright as we continue to build on the progress since 2014 and we should all look forward to an exciting and successful 2017/18 campaign."