Vikings co-owner and president Mark Wilf called O'Connell "a strong leader, an innovative coach and an excellent communicator" in a statement issued from the team.
"Kevin played the game at all levels, which gives him a unique connection to players, and he is highly respected throughout the league," he said.
Co-owner and chairman Zygi Wilf lauded O'Connell for his "football IQ" and said he "is passionate about preparing, developing and communicating with his players."
"Beyond the Xs and Os, everyone we spoke with throughout this search process spoke highly of Kevin's ability to motivate players, coaches and staff," he said.
The Vikings plan to hold a news conference to introduce O'Connell as their new coach Thursday.
O'Connell, 36, becomes the second-youngest current NFL head coach, behind his now-former boss, Sean McVay, also 36 but eight months younger.
Minnesota had not been permitted to announce that it was hiring O'Connell while the Rams were still playing in the postseason. Los Angeles' season ended Sunday, culminating with its Super Bowl LVI victory.
O'Connell was one of four candidates to receive a second interview with the Vikings, along with Rams defensive coordinator Raheem Morris, then-Giants defensive coordinator Patrick Graham and Michigan head coach Jim Harbaugh.
"As I spent time with Kevin during the interview process, it became immediately clear he will be a great partner in building this football team for long-term success," Vikings general manager Kwesi Adofo-Mensah said in the statement. "He understands what is critical to leading a team that wins at the highest level, and he has an ability to simplify the complex, which will help us in problem-solving in all football areas. He is a genuinely positive person who will help create a high-energy internal environment."
O'Connell is expected to take Rams tight ends coach and passing game coordinator Wes Phillips with him to Minnesota as his offensive coordinator or passing game coordinator, league sources have told ESPN's Adam Schefter. Rams running backs coach Thomas Brown also will interview with the Vikings for their offensive coordinator job, per Schefter.
The Vikings also have their defensive coordinator for O'Connell's staff, officially hiring Ed Donatell on Thursday. In addition, veteran defensive coordinator Mike Pettine has been added as assistant head coach.
O'Connell becomes the second coordinator in two years on Sean McVay's staff to land a head-coaching job, joining former Rams defensive coordinator Brandon Staley, who was hired by the Chargers last year. He is the fourth current NFL head coach who worked under McVay. The Bengals' Zac Taylor and Packers' Matt LaFleur are the others.
O'Connell replaces Mike Zimmer, who was fired last month after eight seasons. The Vikings finished with an 8-9 record and missed the playoffs for a second consecutive season.
"This is a franchise with a deep history and a tremendous fan base," O'Connell said in the statement. "Having been on the visitors' sideline at U.S. Bank Stadium and experiencing that environment as an opposing coach, I am excited to now have the support of these passionate Vikings fans. I am committed to working tirelessly to bring them a Super Bowl championship."
O'Connell finished his second season as the Rams' offensive coordinator since joining the franchise in 2020. Before that, he was with Washington for three seasons, as offensive coordinator in 2019, passing game coordinator in 2018 and quarterbacks coach in 2017 when he supervised current Vikings starter Kirk Cousins.
After dabbling in broadcasting, his first NFL coaching job came with the Browns in 2015 as their quarterbacks coach. The following year, he worked on special projects for the 49ers, when he befriended Adofo-Mensah, the recently appointed GM of the Vikings who was in research and development for San Francisco.
He attempted only six passes for the Patriots as a rookie, the only regular-season attempts of his career, and he was released the following year.
The Associated Press contributed to this report.