Uchiha Feiyu ultimately agreed to become Orochimaru's disciple. Of course, this disciple was no weaker than Orochimaru himself. Rather than master and disciple, they were allies to some extent.
For one thing, once Orochimaru became Uchiha Feiyu's teacher, he would naturally receive a share of Uchiha Feiyu's achievements. Not to mention that Uchiha Feiyu's current strength would be of tremendous benefit to Orochimaru's bid for the Hokage Position.
Don't say Orochimaru had no interest in becoming Hokage and only wanted to conduct research. Didn't the Second Hokage, Tobirama Senju, conduct research while serving as Hokage? What he developed was no inferior to Orochimaru's work.
Having a major ninja village backing one's research was an enormous boost. Otherwise, Orochimaru would not have needed to stay in contact with Danzo after defecting, much less establish Otogakure.
Of course, Uchiha Feiyu did not think Orochimaru could truly get what he wanted. After all, everyone knew that in Konoha, becoming Hokage was not about how much merit one had earned, but whether one listened to the Third Hokage.
Still, taking this opportunity to get some benefits from Orochimaru was a pretty good deal.
After becoming Orochimaru's disciple, Orochimaru was not stingy. He immediately gave Uchiha Feiyu two ninjutsu: the Orochimaru-Style Body Replacement Technique and Summoning Rashomon.
These two ninjutsu were not particularly high-ranked, but both were extremely practical. The Orochimaru-Style Body Replacement Technique could be called the finest of all Substitution Techniques. Although it consumed a great deal of Chakra, it could render any damage meaningless.
As for Summoning Rashomon, it was an exceptionally rare and powerful defensive ninjutsu that could even block a Tailed Beast Ball. Its defensive power far surpassed ordinary Water Release or Earth Release defensive ninjutsu.
Having obtained the ninjutsu, Uchiha Feiyu quickly returned to his Training Ground to practice.