Eroge no Sao Oji-san ni Tensei Shite Shimatta

One day, a dull man who loves erotic games dies from overdoing his wanking. He was reincarnated into the world of the erotic game “Too Popular VR: Harem in a School for Young Ladies!" However, he was reincarnated into the world as “Sansuke Ganji,” an old man who cuckolds the hero and a total scumbag! The only way to change this fate is with the Magic Ring, a cheat item that can make wishes come true! He tries to find it as soon as possible, but one of the (lusty) heroines in this erotic world has it...!

(Source: Takeshobo, translated)