Saturday's 32-22 win by the Irish in Wellington clinched a historic first series win in New Zealand and moved them top of the world rankings.
"If we played New Zealand again next week, we'd probably beat them again," said ex-Ireland coach O'Sullivan.
"But I think, come the World Cup, things will even up again."
"There is a lot of water to flow under the bridge between now and next year," he added.
Ireland's latest victory over the All Blacks was their fifth win in eight contests against the three-time world champions.
Speaking on BBC Radio Ulster's Sportsound Extra Time, O'Sullivan, who coached the Irish between 2001 and 2008, added: "Everyone gets more focused for the World Cup, but I do think we're right up there.
"At the moment we are the better team. There's no debating that.
"So it's a very unusual place for New Zealand to be and a very unusual place for Ireland to be, but we're number one in the world today for a very good reason."
Despite his caution over how the World Cup may unfold, O'Sullivan believes Ireland's 2-1 series win will stand Andy Farrell's team "in good stead" for next year's campaign in France.
Prior to the last World Cup, impressive Ireland form saw the Joe Schmidt-led outfit become the world's top-ranked team a year before the tournament began in Japan.
Yet they disappointed once more on the biggest stage as they exited in the quarter-finals after a 46-14 hammering by the All Blacks.