Ireland, who entered the World Cup with semi-final aspirations, flopped abysmally by crashing out at the group stage - nine points adrift of Pool D leaders Argentina.
The Irish Rugby Football Union's inquest into the debacle is nearing its conclusion and O'Sullivan admits it has been a painful process.
But the 49-year-old insists the Barbarians job has begun the healing process.
"For Ireland the World Cup was hugely disappointing. We failed to produce anything that vaguely resembled our form during the season," he said.
"Since then it has been a tough time trawling through everything that happened.
"That process has nearly come to an end and the Barbarians offers me an opportunity to get back on the horse.
"It feels good to be back on the horse. You always ask hard questions and the hardest questions you ask are of yourself."
O'Sullivan admits Ireland will be looking to redeem themselves with a strong RBS 6 Nations.
"One thing with Ireland is you can't afford to be complacent," he said.
"I don't see there being a huge number of changes in the personnel. We certainly have something to prove in the Six Nations."
O'Sullivan is enjoying the different challenges presented by coaching the Barbarians.
"Coaching the Barbarians is a unique honour and it gives me the opportunity to work with some hugely talented people," he said.
"It's very different to coaching Six Nations or international level.
"You just have to act as a conduit between these players - a kind of facilitator. In many way with the Barbarians less is more."