Just to put it in perspective both Jeff Fisher & Jason Garrett had better records in the regular season & the playoffs.
Bill O'Brien (Texans)
Season: 52-48-0 .520
Playoffs: 2-4 .333
Jeff Fisher (Oilers\Titans)
Season: 142-120-0 .542
Playoffs: 5-6 .455
Jason Garrett (Cowboys)
Season: 85-67-0 .559
Playoffs: 2-3 .400
I feel like I am being forced to defend OB here, which I have no energy to do, but there is a significant sample size difference between all three of these coaches. Sometimes a single number doesn't tell the whole story.
OB coached 6 full seasons, having a .541 winning percentage, a winning record in 5 of those seasons, and making the playoffs four out of the six seasons.
Fisher coached 16 full seasons, having .542 winning percentage, a winning record in 6 of those seasons, and making the playoffs six out of the 16 seasons.
To dig a little deeper into Fisher, over a five year span from 1999-2003, he was 56-24 winning at a .700 clip, making the playoffs four out of the five seasons with a Super Bowl appearance. The other 11 seasons outside of this 5-year range he was at .472 with only two winning seasons and two playoff appearances.
Let's also not forget his time as the Rams coach for 4 full seasons where he had a .414 winning percentage, no winning records and no playoffs, putting his entire coaching winning percentage at .512 with more losing seasons than winning seasons.
Garret coached 9 full seasons, having a .555 winning percentage, a winning record in 4 of those seasons, and making the playoffs three out the nine seasons. He was the absolute king of the 8-8 season.
OB (Texans) with 5 winning seasons in 6 years and four playoff appearances, Fisher (Oilers/Titans) with 6 winning seasons in 16 years and six playoff appearances, Garrett (Cowboys) with four winning seasons in 9 years and three playoff appearances.
Average? Mediocre? It's a good description for all three. They all had a 4-12 season on their resume. But as a percentage of seasons above .500, OB was better than both. I would give Fisher the overall nod ONLY because he had a Super Bowl appearance, but outside of that 5-year period he was trash.