There's a lot of confusion about this because technically the US is supposed to be a democracy, whereas Russian oligarchy is more formal, explicit, and intended and less behind-the-scenes, even though Russia has sham elections.

So there's the political system a country is supposed to have, according to its laws and self-image, and there's how that system pans out in practice. A country can be legally democratic but functionally oligarchic because of the impact of capitalism or populism.