In chapter 965 Orochi’s rise to power is shown.
Orochi conspired with an elderly woman, a previous user of the Mane Mane no Mi, to become Shogun, and she revealed to him the history of the Kurozumi Family.
They were one of the five families that served the Kozuki Family as daimyos, along with the Shimotsuki Family, Amatsuki Family, Uzuki Family, and Fugetsu Family.
As Sukiyaki’s father did not have a child at that time, Orochi’s grandfather plotted to take over Wano by poisoning the other daimyos and eliminating his rivals. After the birth of Sukiyaki, his plan was ruined and exposed, and he was sentence to commit seppuku.
The family’s name was tarnished and they lost their status.
The elderly woman had the Yamata no Orochi fruit in her possession and promised to give it to Orochi once he became shogun.
So we finally got to see how long he had been scheming. Orochi got manipulated into stealing a title that was never rightfully belonging to his clan.
Orochi horded gold by stealing from Yasuie and borrowing from Oden.
Using her ability, the elderly woman posed as Oden, and Orochi was placed under Sukiyaki’s employment.
Sometime later, the woman posed as Sukiyaki, who seemingly became ill and named Oden as his successor, and having Orochi serve as a replacement until Oden’s return.