O'Brien, who two years ago replaced Joe Paterno at a Penn State football program that was shaken by the Jerry Sandusky child sex abuse scandal, takes over for Gary Kubiak.
"There's a lot of pieces in place here. 'Quick turnaround,' 'rebuilding' -- I'm not into labels," O'Brien said during his introductory news conference. "I do believe this is a strong roster of players. It'll be fun to evaluate it."
O'Brien, a former offensive coordinator with the NFL's New England Patriots, said he will meet with every member of the Texans' coaching staff on Saturday.
Houston won the AFC South division with a 12-4 record in the 2012 season and had lofty expectations coming into the current campaign. But a string of losses cost Kubiak his job, making him the first casualty of the season.
After opening the 2013 season with two wins, the Texans went on to lose the next 14 games, the last three under interim coach Wade Philips.