The Ohara Incident is the event that took place in 1502, twenty-two years ago. Iceburg was the first to mention it, but Robin told him that he did not know anything about it. This event took place in Ohara in the West Blue. This is where Nico Robin was born and lived for eight years. This event explains who Robin is.
Some time after the Void Century, the World Government outlawed the research of Poneglyph. Because of this, countless people who tried to study the hundred years were killed. Ohara was an island home to a group of archeologists researching the Void Century who believed that they managed to evade the World Government's eyes.
In 1496 six people along with Nico Olvia, Nico Robin's mother, went on a journey to complete her late husband's work of studying the Void Century. However, after six years, Olvia and her group were caught by the Marines. All of the archaeologists but Olvia were killed while defending her. The Marines managed to look at the IDs of the dead Oharans and found out that they were from Ohara.
Because of this, the World Government decided that they would have to destroy Ohara with a Buster Call. Admiral Sengoku ordered Vice Admiral Jaguar D. Saul to be one of the five Vice Admirals to lead the Buster Call. However, Saul disagreed with him and, after talking to her, freed Olvia from the Marines and deserted. Saul was soon shipwrecked in a storm and coincidentally drifted to Ohara. Robin then found Saul, who just woke up without knowing where he was, and he befriends Robin who happens to be Olvia's daughter.
They soon became fast friends. Robin would steal bread and jam to feed Saul. Before departing he asked where he was and Robin answered Ohara. This shocked Saul and he paniced, telling Robin that Marine warships were heading to Ohara and he had warn her people. At first Robin thought he was lying but he told her that he was telling the truth as he was from the Marines as well. Shocked, Robin ran off while continuing to panic.
Meanwhile, Olvia arrived at the Tree of Knowledge,tried to warn everyone that the Marines were coming, and told them that they had leave, but Clover refused to leave, saying that the books were their treasures and the archeologists would protect them. Olvia then heard of the CP9 arriving on shore and decided to take matters on her own hands. She grabbed a rifle and fled to where CP9 was, passing Robin who was running to the Tree of Knowledge, passing each other.
Olvia arrived at CP9's location and tried to stall them by shooting at Spandine's waistcoat. Olvia tried to tell them that she was responsible for her actions and Ohara was not involved in this. Spandine told her that her efforts were worthless, and Ohara was going to be destroyed, much to Olvia's shock. Spandine then ordered Laskey and an unnamed agent to take her down, which he used Rokushiki to do so.
Meanwhile, Robin arrived at the Tree of Knowledge and tried to warn Clover and the others but they refused to leave. Suddenly, the door burst open and the agents swarmed in, telling everyone to go outside as they began to ransack the library much to the scholars' shock. Clover tried to reason with them but one of the agents told Clover not to tell him how to do his job.
Eventually, the agents found a hidden basement where the scholars were secretly studying a poneglyph. Knowing this, Spandine immediately contacted Marine Headquarters, which led to an argument between Clover and the Gorosei and ended with Clover getting shot and seriously wounded. After the argument, the Gorosei ordered the Marines to activate the Buster Call. They shot the Tree of Knowledge, setting it on fire as the scholars desperately tried to put it out and save the books.
Meanwhile among the chaos, Robin was finally reunited with Olivia. At first Olivia tried to lie in order to save Robin from being labeled as a criminal's daughter; however, she finally gave in and hugged Robin. The reunion was short lived as the marines relentlessly bombarded the island with their warships. Saul came and tried to save them but Olivia decided to stay and told Saul to leave the island with Robin, telling her to live.
Running towards the beach, Saul was met by some of the marine ships, which shot at them, angering Saul. He destroyed a few of their ships until he was stopped by Kuzan. After a short argument about justice that was interrupted by Vice Admiral Sakazuki ordering his men to fire on the evacuation ship, Saul tried to flee with Robin, but Kuzan decided to end it by freezing Saul using his devil fruit powers. Robin, horrified by what happened, ran from Kuzan, only to be relentlessly shot by the marines. She arrived at the beach on the other side of the forest where Kuzan had a small boat for Robin to escape and an ice path leading to the next island. He told her he was letting her go but warned her that he wasn't her friend and that he would go after her if she became a criminal. Robin then took the boat and escaped Ohara.
This incident shook the world to the point Ohara was no longer drawn on the map. Robin was given a bounty of 79,000,000 and was demonized by the World Government. To cover up the Buster Call and the real reason for Robin getting a bounty they said she sunk five marines ships and the Ohara clan was seeking out forbidden knowledge to destroy the world. Robin would remain on the run for twenty years. Every group she attempted to join would later betray her and she would escape. She would eventually end up with the Straw Hat Pirates. Robin was still determined to find out what happened during the Void Century and continued to study the Poneglyphs.