O'Brien warmed up with the Test team, went through every single pre-match drill and even sat pitchside, but he wasn't officially part of the matchday squad and therefore knew he wouldn't be getting any game time on the greatest stage of all.
Fast forward 72 hours and the Leinster star has a chance to move on from what must have felt like a slightly surreal situation at the Suncorp when he starts on the openside flank against the Rebels. A positive result and a strong individual display could see him handed the chance to get one step closer to the action in the second Test, something that he and every single member of the squad desperately craves.
"Yeah, it was difficult enough. I had to prepare like I was going to play just in case anything did happen. Doing the warm-up and feeling that buzz and then sitting on your chair for the rest of the match is disappointing, obviously," said O'Brien.
"As Lyds (Dan Lydiate, who captains the Lions this evening) has said, we all have a chance to stake a claim again and go out and show what we can do. The coaches are telling us this and we have to put our trust in them as well.
"We can only look after ourselves. We have another match to play and there is no point going out thinking we haven't got a chance. That will only affect your performance so you have to do what you can do and do the best you can do in every opportunity you get in a Lions jersey."
One man who won't get that opportunity again on this tour is O'Brien's fellow Ireland international Paul O'Connell after the veteran second row was ruled out of the three remaining matches with a fractured arm.
The former Lions captain is undecided on whether to stay on tour or to head home but O'Brien admits that losing the lock forward is a big blow either way.
"I think just his presence," was O'Brien's response when asked to sum up just why it was so disappointing to see the 33-year-old's playing participation brought to a premature end.
"He has a lot of experience, he's a big physical man, he has won lots of trophies in his career and he has been playing at the top of his game for so long now.
"But off the field as well he is someone the younger fellas do look up to. You can confide in him and talk to him and he will be missed all round."