The Immorals (2006)

An evil politician Arimiya Kamoshige has been a local governor for 40 years. Abusing his power, he has gained wealth and women. The protagonist's father had managed Arimiya's black money as a right-hand person, but later, he was expelled falsely charged with a crime. He got discouraged and called his runaway son, Tatsuya. He asked Tatsuya to take revenge on Kamoshige. Soon after his father died. Kamoshige hired Tatsuya as the third secretary. However, Kamoshige also dies 3 months later. Just before his death, he changed the passcode of a safebox which contains hidden black money. Then, the power balance of Arimiya Family changes. The person who obtains the hidden money will become the next ruler. While Arimiya Family is in conflict for the hidden money, Tatsuya begins his revenge.

(These OVA's are based on the erotic game by Silky's.)