When asked whether the absence was excused, O'Brien did not respond either way.
"It's between Ryan and I," O'Brien said. "It's a personal issue."
Mallett was at the facility later in the day for meetings.
Asked whether Mallett will be back for Friday's practice, O'Brien said, "I should think so."
The Texans chose Brian Hoyer as the team's starting quarterback on Monday after a competition that lasted through the Texans' second preseason game. Hoyer started the preseason opener, and Mallett started the second game of the preseason.
Mallett said Tuesday he was disappointed and a little bit angry after losing the competition.
"I don't agree with it, but it's for the best of the team, so I'm just going to wait till my turn gets called," Mallett said. "And I mean, at some point it's going to happen this year."