The 28-year-old reportedly received a three-year, $16.4 million deal, with the third year being a player option.
A key reserve with the Toronto Raptors last season, Patterson could move into a starting role for the Thunder, who this week saw forward Taj Gibson leave for the Minnesota Timberwolves as a free agent.
Patterson is the second significant offseason addition for Oklahoma City, which picked up Paul George in a trade with the Indiana Pacers. That deal saw another forward, Domantas Sabonis, exit the Thunder for the Pacers.
In 65 games (eight starts) for Toronto last season, Patterson averaged 6.8 points and 4.5 rebounds. In 10 playoff games (one start), he produced 3.4 points, 2.0 rebounds and 2.0 assists per game.
A first-round pick by the Rockets in 2010 out of Kentucky, Patterson spent 2 1/2 seasons in Houston. He played parts of two seasons with the Sacramento Kings before joining the Raptors for in the 2013-14 season.
Patterson has appeared in 477 career games, making 73 starts, and he has averages of 7.9 points and 4.7 rebounds.