What is Uncle Oscar's role in "The Rocking-Horse Winner?"

Uncle Oscar is first used for purposes of exposition through dramatic dialogue. He finds out that Paul is winning money on the horses and that he is in partnership with the family gardener whose last name is Bassett. Both Paul and Bassett explain to Uncle Oscar, in extensive dialogue involving many questions and answers, that Paul picks the winning horses, and they tell him to his amazement how much money they have won so far. Paul does not need Uncle Oscar to handle the betting or to keep the money, but he wants to give his mother five thousand pounds out of his winnings without having her know where the money came from. Uncle Oscar can handle this, whereas neither Paul nor Bassett would know how to do it.

They managed it very easily. Paul, at the other's suggestion, handed over five thousand pounds to his uncle, who deposited it with the family lawyer, who was then to inform Paul's mother that a relative had put five thousand pounds into his hands, which sum was to be paid out a thousand pounds at a time, on the mother's birthday, for the next five years. 

Uncle Oscar handles other such matters for Paul, but his main functions as a character are fulfilled when he has been told all about Paul's and Bassett's winnings and has shown that he knows how to transfer money from Paul to his mother without her knowing the source of her sudden bounty. Uncle Oscar is a suitable character because he is a horse player himself and because he is thoroughly trustworthy with the money and with Paul's secrets. Uncle Oscar is also allowed to take Paul to some of the races--a fantastic experience for a boy so captivated by horses and horse-racing. 

One thing Paul keeps secret from everybody is that he is getting the names of the horses destined to win the important races by riding his rocking horse. If he rides long enough and hard enough, the name of the horse will come to him intuitively. Many horse players develop superstitions about picking winners. There is just enough plausibility to Paul's unique "system" to make the reader believe in the whole story. Uncle Oscar has the last word after Paul dies of what might be a brain hemorrhage brought on by his frantic efforts to pick the winning horse in the forthcoming Derby. 

"I never told you, mother, that if I can ride my horse, and get there, then I'm absolutely sure - oh, absolutely! Mother, did I ever tell you? I am lucky!" 

"No, you never did," said his mother. 

But the boy died in the night. 

And even as he lay dead, his mother heard her brother's voice saying to her, "My God, Hester, you're eighty-odd thousand to the good, and a poor devil of a son to the bad. But, poor devil, poor devil, he's best gone out of a life where he rides his rocking-horse to find a winner."