Westbrook, who spent his first 11 seasons with the franchise, will "always remain part of the Thunder family," Presti said.
"Russell Westbrook is the most important player in the brief history of the Oklahoma City Thunder," Presti said in a statement.
"He has left an indelible mark on this team, city and state. None of us could have anticipated the player he has become, and we are all deeply proud of what he has contributed to the success of the franchise and to our community."
“Russell Westbrook is the most important player in the brief history of the OKC Thunder. He has left an indelible mark on this team, city and state.” -GM Sam Presti pic.twitter.com/53SbwyAIXl— OKC THUNDER (@okcthunder) July 17, 2019
Westbrook was the second top-level player the Thunder sent away this offseason. They moved Paul George — who finished third in the MVP voting in 2018-19 — to the Los Angeles Clippers and received five first-round selections, two pick swaps, Shai Gilgeous-Alexander and Danilo Gallinari in return.
The Thunder then dealt forward Jerami Grant to the Denver Nuggets for another first-round pick.
"We recently had conversations with Russell about the team, his career, and how he sees the future," Presti said. "Through those conversations we came to the understanding that looking at some alternative situations would be something that made sense for him. As a result, and due to his history with the Thunder, we worked together to accommodate this.
"Our ability to have these types of conversations and work so closely with Russell and his agent Thad Foucher is only possible because of the depth of the relationship that has been built over the last 11 years."
The deal reunites Westbrook with James Harden in Houston. The pair played together with the Thunder from 2009-12.
The Thunder could look to trade veteran guard Paul again before the start of 2019-20.