The 47 Ronin knew that before they could launch the attack, they would have to put Kira guard down. They decided to split up to avoid suspicion and to start to plan their move on the well protected Kira. They gathered in and around Edo, disguised as monks, merchants and even drunkards to get information on Kira in order to help them penetrate into his mansion.
Only their mission matters; one of the Ronin even married the house builder's daughter to get a hand on the plans of Kira's mansion.
Others Ronin were secretly transporting to Edo weapons as well as armors and equipment by boat during the night.
Oishi knew that by raiding on Kira's mansion, the 47 Ronin were going to their death, not only they could die during the attack, but if they succeed, the Shogun will probably want them dead. He went to his wife and divorced her without telling her anything about his plan. He sent her away to live with her parents, along with their two younger children. Oishi's oldest boy, Chikara decided to stay an fight alongside his father. He just sixteen years old.
Fearing a plot growing against him, Lord Kira sent spies to watch the Ronin, particularly their leader, Oishi. It was essential that their plans was not discovered.
Knowing that Kira sent spies after him, Oishi went to Kyoto and began frequenting brothels and taverns earning reputation as a drunken gambler. This way, Oishi Kuranosuke was trying to deceive Kira's spies, but on the inside, he was consumed by the idea of revenge.
Agents of Kira reported that the Asano's retainers, now away from the warriors life, became merchants and drunkards. With time, Kira started to believe that Asano's Ronin did not have the courage to avenge their master and that he was safe from them. The masquerade worked just as planned, Kira let down his guard.
At a secret meeting, Oishi and the other Ronin decided that the time had come to raid against Kira mansion and avenge their Master's death. The moment to strike is perfect.