The Limerick manager had seen his side dominate proceedings in the Gaelic Grounds and lead by three points as the end of the 70 minutes approached, then within 60 seconds, the Rebels struck for two goals.
"I felt the better team lost today but that's football - that's sport and it is a cruel lesson," said the Limerick manager after the 2-9 to 0-12 defeat.
"I looked at my watch and I said that there is two or three minutes to go and all we have to do now is not concede a goal and then what happens - well that's cruel, but that's sport."
While Cork progress to a Munster final against Kerry it is the qualifiers for Limerick.
"The lads have indicated inside (in the dressing room) that they are going to bounce back and they have won back respect for Limerick football today and they have shown that they are as good as any team in the country," explained O'Sullivan.
"These lads are a serious outfit and we have a good panel and we will play in Croke Park this year. We are taking the qualifiers seriously; we are not sitting down."
Limerick will be back in action on July 19 in round one of the qualifiers.
Sixteen teams will go into the draw in the coming weeks and with Tyrone, Donegal, Meath, Cavan, Kildare and Monaghan among the likely opponents, the backdoor route will be anything but easy.
"They showed that they are a serious outfit and they will take beating," said the Limerick manager of his side.
"We are building and this is what we have been building on for three years. We are now playing the type of football we want to be playing.
"I am proud of them because I felt that they did exactly what we wanted them to do and they stuck to the game plan to the very last.
"We know these guys and they are a serious outfit, but the people of Limerick must get behind them and have a bit of pride in them."
Meanwhile, the Limerick senior hurlers will announce their team tomorrow night for Sunday's Munster championship semi-final against Clare.
Richie Bennis' side have a late injury worry over Wayne McNamara who picked up an injury in training last week.