Celtic were utterly dismantled against Paris Saint-Germain in midweek.
Brendan Rodgers's side were subjected to a mauling by Paris Saint-Germain earlier in the week as the French giants kicked off their Champions League campaign with a 5-0 demolition of the Scottish champions at Parkhead.
Before kickoff, there was a feeling that Celtic might just roll back the years and pull off another major shock on the European stage in front of their adoring fans, but everything pretty much went to plan for big-spending PSG who never looked fazed.
The Hoops won every domestic competition in Scotland last season without losing a game, and O'Brien, who spent three years on the green half of Glasgow, admits it was good to see his old side get an insight into how every other Premiership club feels against them.
“I said that to my friends. I am in a group chat with my friends at home and they were saying: ‘It’s not good enough, we are not getting near them’," he told The Scotsman.
“And I just said: ‘Well, this is how everyone else feels when they play against Celtic’.
“It’s nice to see Celtic maybe getting a little taste of that and realising how everyone else feels."
O'Brien should probably guess again if he thinks that such a heavy defeat would prompt Celtic to grow more sympathetic toward the also-rans in Scotland's top flight.