Oligarchy is rule by the few, and if the few are thieves, the government will be kleptocratic too, as it is in Russia.

The US is supposed to be democratic, which is the opposite of an oligarchy. The question is whether the democracy is healthy and self-sustaining, or whether it's eroding due to pressures from capitalism and from human nature. Democracy might be like monogamous marriage in that they take work to maintain. They go against the natural grain.

If a democracy is sufficiently corrupted, the country might be nominally democratic but functionally oligarchic (and even kleptocratic), in that the few have more political power than the many. Look at how gerrymandering, the Electoral College, and the filibuster, for example, exloit the loopholes and rob the majority of their control over the government.