Leinster currently sit in fifth position, eight points adrift of leaders Glasgow Warriors.
Kearney, who stands at 6ft 5in, has represented Connacht 15 times this season and O'Connor is happy to see him return to the province he represented through the younger age groups.
"We are delighted to secure the services of Mick Kearney," he said.
"Leo Cullen and Marco Caputo have been very impressed with his progress and he has done well over the last number of seasons with Connacht and we think that he will contribute hugely to the set up here.
"He is also a young guy having just turned 24, so we very much see him developing as a player and I think he will benefit hugely from working directly with players of the experience of Devin Toner and Mike McCarthy."
Kearney was equally enthusiastic about his move but insisted his focus was on winning as many games as possible with Connacht between now and the end of the season.
"I am very happy to have signed on with Leinster for the coming season and I look forward to working hard to add to a proven winning environment and culture," he said.
"Having said that, I'm very grateful to Connacht and all the staff and coaches over the years for the opportunities afforded to me since I made my debut in 2011.
"I'm just focused now on finishing out the season with Connacht as best as I can and helping them to secure the top six position that I feel our efforts deserve."
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