A disappointing 2005 championship and poor November series saw Ireland's head coach come under increasing pressure with his tactical nous and team selections in the firing line.
But O'Sullivan would rather focus on the steely edge developed by his side during an arduous 2006 Six Nations campaign.
"I knew at the start it would be a difficult season for us," he said.
"The November series was hard for us because we were playing Australia and New Zealand without Brian O'Driscoll and Paul O'Connell.
"We battled through the autumn and then knew the Six Nations would be full of tight games. We ended up going to Twickenham with a shot at the title.
"It wasn't easy but when faced with adverse situations and when things aren't working out for you, you can become stronger or weaker.
"We have dug deep and taken the rough with the smooth and come through a better team than six months ago.
"It's not about vindication, it's always been about trying to develop a better rugby team. I said at the start we're going through a transitional phase.
"The team that played against England is almost half the team which finished in Cardiff last season. That's transition and we're not done yet."
The Irish boss insists there are signs his blueprint of more ambitious rugby is being realised on the pitch and knows the true test of progress will come during the summer tour to New Zealand and Australia.
"Part of the transition is knowing what you want to do with the football. It's also about having confidence in the team to execute," he added.
"When we started out in the autumn it didn't go so well. We made mistakes and were punished harshly by New Zealand and Australia.
"But we were going in the right direction and are playing better rugby now. It's always work in progress and we are very happy with the Six Nations.
"We've taken a step forward. There are another three Tests at the end of the season and hopefully we'll become a better team during those."