Aaron Wilson of the Houston Chronicle first reported the news. Defensive coordinator Romeo Crennel will serve as the team's interim head coach for the remainder of the season.
The Texans hired O'Brien in January 2014 after he spent two years as head coach of the Penn State Nittany Lions.
The 50-year-old Massachusetts native enjoyed a promising start to his time in Houston. He went 9-7 in each of his first three seasons, the latter two of which resulted in playoff appearances. The team advanced to the divisional round in 2016 with a postseason win over the then-Oakland Raiders.
O'Brien and Co. haven't been able to improve that playoff performance in recent years amid rising championship expectations, though.
The Texans finished 4-12 in 2017, but they had a few legitimate excuses. Quarterback Deshaun Watson saw the blistering start to his rookie season halted by a season-ending ACL injury after seven games, while defensive end J.J. Watt played only five games before a leg injury ended his campaign.
Watson and Watt returned for the beginning of the 2018 season, which helped get the franchise back on track. They won the AFC South with an 11-5 record but lost to the Indianapolis Colts in the AFC Wild Card Round.
While the Texans won another division title with a 10-6 record last season, their championship quest fell short again. They beat the Buffalo Bills in the Wild Card Round, but the Kansas City Chiefs overcame a 24-0 deficit in the divisional round to beat the Texans 51-31. That loss led to serious backlash against the head coach, who made several questionable decisions during the Chiefs' comeback.
O'Brien went 2-4 in six playoff games and leaves Houston with a 52-48 regular-season record.
He received further criticism for a blockbuster trade of star wide receiver DeAndre Hopkins to the Arizona Cardinals in the offseason, which included rumors about his discussions with Hopkins before the deal:
Along with his time leading the Houston and Penn State staffs, his resume includes nearly two decades of experience as an assistant at the pro and collegiate levels. That includes five years with the New England Patriots in various offensive coaching roles.
As a result, O'Brien should have the opportunity to slot back in as an assistant somewhere if he's unable to find another head coaching position.