The Killarney woman had the 20km walk bronze medal snatched from her by Italian Erica Alfrida with the finish almost in sight but managed to set an Irish record of 1:28.46.
In world terms it was a sensational improvement on her 10th placing in the Sydney Olympics with just a couple of the top performers in the world now operating outside of Europe.
And she practically had the bronze medal around her neck as she approached the Olympic Stadium.
However, it was taken away less than 500m from the finish of what was a gruelling and, at times, ferociously fought event.
For the Irish fans who lined the route in big numbers and then dashed back to the stadium to witness the finish it proved to be a cruel ending to what had been a spellbinding duel between two courageous women.
But it should not be allowed to overshadow this brilliant performance from the Killarney athlete.
O’Sullivan was the star performer in the race for the bronze medal once the leading pair, Olimpiada Ivanova and Yelena Nikolayeva of Russia, had powered clear.
Ivanova, the world champion, took the title in a championship record of 1:26.42 from Nikolayeva, the 1996 Olympic champion.
Nikolayeva had to let her compatriot go midway through the race and finished second in 1:28.20.
It appeared she might offer a major challenge to Ivanova, the world record holder, but adopted a more cautious approach when she received a warning that probably caused her to reflect on her disqualification in last year’s World Championships.
When Ivanova stormed out of the stadium in typical fashion a chasing group of 18 formed in her wake.
Gillian O’Sullivan had established herself as a key figure by the 5k mark with Ireland’s other representative, Olive Loughnane from Loughrea, just off the back.
“It was a tough race,” O’Sullivan said, recalling how some big names made their departure.
“The Russians were gone from the bell and then there was a chasing group but they got less and less.”
“I was surprised when the Norwegian girl (Kjersti Platzer) went and then I was even more surprised that the Spanish girl (Maria Vasco) actually dropped out. That was very surprising because I expected her to go for a medal.”
O’Sullivan battled for control of the group with Platzer, until her surprise departure, and tAlfrida, passing 5k in 22:29 and 10k in 22:48 with the Italian now her only serious challenger for the bronze.
“I did a lot of the leading at the front of the group. You just could not let them go and at 10k I felt really good,” she said.
“Soon it came down to three of us and then there were just two.
“There was a tough race between me and the Italian girl. She would lead and then I would lead. We kept at it and then I managed to get clear.”
The race was decided around a 2k circuit and going out for the final time Gillian enjoyed a lead of almost 20m.
“But I always had that feeling that she would come back,” she said.
“I kept pressing forward and I gained a lot on her but then I felt that I was really hanging on.
“My breathing was gone and my legs were wobbling.
“I knew there was danger about but I did not think she was actually that close.
“At around 18km I felt I was really working and I felt I had about 10m on her.
“I knew I was at my max and it was just a matter of how long I could hold it that and it hit me all of a sudden with 1km to go. My legs were wobbling. Then, with just about 500m remaining she came past me.”
Few expected the change in this struggle for third place at such a critical point. The Italian dug amazingly deep into her reserves to reel in the Irish woman and claim the bronze.
Olive Loughnane also gave a sterling performance yesterday, battling her way back from 21st place to finish 13th in 1:33:08, her second fastest time ever. She also finished 13th in the world championships in Edmonton last year.
“In fairness, there is nothing wrong with two in the top 13,” she said.
“We had a walker fourth and Robert Heffernan was eighth, and with Sonia’s second last night we are doing very well out here.”