9:59 AM PT -- Bill O'Brien just kicked off his Friday press conference by saying he was sorry for his epic rant ... telling reporters, "I used inappropriate language and I just want to apologize for that."
Later in the conversation with media members, O'Brien added, "I'm sorry that it happened."
"Screw you, motherf**ker! You suck too! Hey! You suck too, motherf**ker!!!"
That's Texans head coach Bill O'Brien going absolutely ballistic on a heckler during the Broncos game earlier this season ... and the EPIC rant was all caught on video!!!
Houston fans were pretty pissed off at their team during their Dec. 8 game -- when the Texans were down 31-3 at half -- and one fan decided to let O'Brien hear it in the team's tunnel.
As the coach was on his way back to the locker room with star quarterback Deshaun Watson -- you can hear a man clearly yell "YOU SUCK!" at the guys.
That's when 50-year-old Billy O went OFF ... turning toward the fan and unleashing a tirade for the ages!!!
"You suck too!" O'Brien screamed. "F**k you!"
You can see in the video, star receiver DeAndre Hopkins tried to break up the scene ... but that didn't stop O'Brien from firing at the heckler, "Go the f**k home, motherf**ker!"
Clearly, there are much happier times in Houston these days ... since that blowup loss -- the Texans have gone 3-1, including a massive playoff win over Buffalo last weekend.
And, of course, the team is now just two wins away from its first-EVER Super Bowl appearance.
Which begs the question ... wonder how the heckler's feelin' now?