Brian Hoyer started Sunday's loss to Kansas City, but Ryan Mallett finished.
In Week 2 at Carolina, we'll find out who starts when the Texans offense trots on the field.
"I'm not going to make a big quarterback announcement today," O'Brien said during a Monday press conference. "You'll have to wait until the first offensive play of the Carolina game to see who the quarterback will be."
Hoyer completed 18-of-34 passes for 236 yards. He threw a touchdown to DeAndre Hopkins, but was also picked off. The Chiefs sacked Hoyer four times, forcing a fumble on one of those.
Mallett, meanwhile, was 8-of-13 for 98 yards over the two possessions he played. He too, threw a touchdown to Hopkins. He also engineered a drive that resulted in a field goal.
O'Brien explained why he's holding off revealing who his starter will be against the Panthers.
"Every decision that I make, I really try to sit down with our staff, organizationally, and make decisions that are in the best interest of the team, that give us the best chance to win," O'Brien said. "That's regardless of position."
Mallett entered the Week 1 contest on Sunday with 6:10 remaining in the game. The Texans trailed 27-9 at that point, but Mallett was able to help close to within 27-20. O'Brien stressed on Monday, that who he chooses to start Sunday is reliant upon a variety of factors.
"It's a big-picture decision," O'Brien said. "It's not just one play or this or that. It's what we think is the best thing that we need to do to go out there and help us beat Carolina."
The players are off on Tuesday, and will hit the practice field from Wednesday through Friday at the Houston Methodist Training Center.
Take a look at photos from Sunday's matchup with the Chiefs.