For what it's worth, both Abramovich and Usmanov have credible recent track records by the standards of western billionaires. Philanthropy, patronage of sport and the arts, Usmanov's made packets off hipster tech companies, and so on.
I was reading Usmanov's biography a few days ago and he didn't even make his cash from voucher privatisation initially. He sounds rather similar to an Australian mining magnate like Twiggy Forrest: loathsome, but little different.
It would be interesting if someone produced a measurable definition of an oligarchy. I bet there are many plausible sounding candidates you could come up with that wouldn't distinguish Russia from the United States.