While the four-year partnership between O'Brien and Smith has produced two AFC South division titles, the relationship between the two is characterized as problematic and could lead to an either-or-situation going forward, according to multiple sources with knowledge of the situation but not authorized to speak publicly.
"It's toxic," one source said. "It's dysfunctional."
Another source speculated that Smith and O'Brien could keep working together but that it would be an "uneasy alliance" that's somewhat untenable.
A third source with knowledge of the situation said he would be "kind of surprised" if the Texans don't make a change.
While O'Brien has always described his relationship with Smith as good, this has been a time of serious uncertainty with a fair amount of anxiety at Kirby Drive as McNair, who likes continuity, contemplates how he'll proceed. Should McNair decide to punt and put off the decision until O'Brien's contract expires next year, sources say the Texans could be maintaining a difficult work environment.
O'Brien is entering the final year of his contract in 2018. If O'Brien becomes available through a firing - O'Brien has stated he won't quit - or via a trade, which would be a complicated scenario that would involve a hefty amount of draft-pick compensation, he would arguably become the top free agent coach on the marketplace with several franchises interested in acquiring the former New England Patriots offensive coordinator and Penn State coach.
Among the many potential destinations for O'Brien, if he becomes available, and jobs become open: Tampa Bay with general manager Jason Licht, who worked for the Patriots previously, Detroit with general manager Bob Quinn, who also worked in New England, Arizona with general manager Steve Keim and Tennessee with general manager Jon Robinson, who also has a New England background.
It would be an ideal time for O'Brien to become a free agent because industry sources predict so many head-coaching positions will be available in this hiring cycle.
"I think Bill is an excellent football coach," said former Texans and Washington general manager Charley Casserly, an NFL Network analyst. "I think if he were a free agent, he would have a job before he left the building and got home."
The Texans have endured a rough year, including experiencing Hurricane Harvey.
"OB can flat-out coach," said an NFL executive, who asked not to be identified when speaking about another team's coach. "I would have him on a very short list of guys I would want to talk with right away if we were in the market for a coach. When Watson was healthy, they had it rolling. When he got hurt, they just weren't the same team.
"What he did with Watson this year and previously with Tom Brady is impressive and I've heard good things about him as a leader. It will be interesting to see what happens with them."
O'Brien said this week that he would be willing to coach the Texans next season, even without a contract extension. That would an unusual, but not unprecedented situation.