"We feel down and we are very disappointed," said O'Sullivan, rueing a glorious chance to have secured a first Grand Slam since 1948.
"But they are the number one team in the world and they are right on top of their game.
"They defended very well and in the second-half they strangled us up front and struck at critical moments. (Mike) Tindall's try especially was a stake in the heart.
"They have seen everyone off in the autumn and seen everyone off in the spring. With that sort of form over the year they must be favourites for the World Cup.
But the coach made sure he emphasised his team's own good form.
He added: "If anyone had told me last August, however, that we would play 11 matches and have won 10 I'd say that's not a bad run.
"We need to be crisper and have better lines of running and we need to be more clinical against the top teams.
"But it has taken England a few years to get to this point. I told the lads to hold their heads high and stick out their chests because they can be proud of themselves."