Mallett will replace Ryan Fitzpatrick, who has started all nine games for Houston this season. Mallett has not attempted an NFL pass since 2012.
"This might be the thing that helps our team," O'Brien said Wednesday. "I might be wrong."
O'Brien said benching Fitzpatrick in favor of Mallett was a "very difficult" decision, but that Mallett's command of the offense and progress in the running system convinced him to give Mallett "his shot."
According to Brian Smith of the Houston Chronicle, O'Brien said Mallett will be given a full chance at keeping the starting job. Fitzpatrick will now back up Mallett, and Tom Savage will continue to be the team's third-string quarterback.
O'Brien also said Mallett has matured a lot and is a "very bright guy" who is highly competitive. Before being traded to the Texans in August, Mallett spent three seasons with the New England Patriots. In 2012 and 2013, he was Tom Brady's primary backup and appeared in four games, completing 1-of-4 passes and throwing an interception.
Fitzpatrick has a 61.8 completion through nine games, throwing for 1,960 yards, 11 touchdowns and eight interceptions in his first season in Houston.
The Texans (4-5) have a bye in Week 10. They face the Cleveland Browns (5-3) on Nov. 16.
- Molly Geary