Saturday's encounter in Wellington will see two of the best back-rows in world rugby collide, with Lydiate, Sam Warburton and Toby Faletau going up against O'Brien, Stephen Ferris and Jamie Heaslip.
And Lydiate, who returns after missing the last two games because of an ankle ligament injury, expects a fierce examination from their Irish counterparts.
"He (O'Brien) is a class player," Lydiate said. "He was European player of the year last season.
"And it isn't only him. Stephen Ferris was on the (2009) Lions tour and Jamie Heaslip is a Lions Test player, so they have arguably the best back row in the tournament.
"It is going to be an interesting game, but I know our back row will be up for it.
"O'Brien is a go-to man, someone who always gets them over the gain-line whoever they are up against. He trucks it up for them and is really effective at it.
"It does take a lot of people to stop him when he has those bullocking runs. It's our job to try to nullify him.
"We must not miss any tackles and as best we can, keep knocking them back. Playing against South Africa and the Pacific islands teams has put us in good stead.
"They are all big blokes, so the boys are all conditioned now to deal with powerful men who take some stopping."
Victory for Wales would see them book a first World Cup semi-final place since 1987, while Ireland have never previously reached the last four.
And both teams have the knowledge they are in peak form for what is likely to prove a gripping contest.
"There is going to be two teams going for it, a lot of blood, sweat and tears by the end of the game," Lydiate said.
"Since the South Africa game (Wales lost 17-16) we've effectively been playing knockout rugby - one slip and we would have been out.
"And the great thing is that while we've been prepared to grind out results, we have also shown we can play flamboyant, awesome rugby to watch.
"It is going to be an even contest that will go down to the wire."
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