OroSasu

OroSasu is a Slash ship between Orochimaru and Sasuke Uchiha in the Naruto series.

Canon
Orochimaru set his eyes on Sasuke because he was the last remaining member of the Uchiha Clan and wanted his Sharingan. He first encountered him in the Forest of Death and gave him the Cursed Seal of Heaven while insisting that he would eventually come for him. At first, Sasuke resisted this at all costs. However, when he proved to be no match for Itachi and grew jealous of Naruto, he decided to leave in order to go train with Orochimaru.

Orochimaru trained Sasuke over the next two and a half years. He eventually prepared to take his body but Sasuke had been aware of this since before he joined him. Sasuke managed to use his Sharingan to absorb Orochimaru instead. During the Fourth Shinobi World War, he found a way to revive him because he began to question what a village is and wanted answers. Orochimaru found a way to revive the four deceased Hokage so that Sasuke could ask questions.

Years later, Sasuke was married to Sakura and had a daughter named Sarada. A man named Shin Uchiha attempted to kidnap Sasuke and Sarada and eventually managed to capture Sakura. Sasuke went to Orochimaru's lair to learn more about him. He explained that Shin was one of his experiments but was acting on his own by then. Orochimaru told him where he was so that he could go kill him.

Fanon
OroSasu is not a very popular pairing in the Naruto fandom. This is mainly due to their age difference and the fact that Orochimaru was using Sasuke in hopes of eventually possessing him. Other pairings such as SasuNaru and OroJira are more popular. However, with the way that Orochimaru had previous acted towards Itachi, some believed that Orochimaru could have had a thing for Sasuke instead of just wanting to use him.