The definition of an oligarch is "someone who is part of a small group that runs a country." (Wiktionary)

So, you can try to claim that Russia doesn't have oligarchs, but if they exist you can't say that they don't run the country.

EDIT: For example, if the country is run by the military as you say, then the top leaders of the military would make up the oligarchy and each of them would be an oligarch.

I'm unsure how any dictator could run a country in the absence of some form of supporting oligarchy.

Sure, but the sanctions are aimed at the rich businessmen not military leaders (who aren't necessarily all that rich). Eg Khadorkovsky was the richest oligarch of Russia, has been jailed and became a major opponent of Putin.

Wonder who are the oligarchs of USA by that definition?

What makes you think you know better than anyone else who is actually running Russia?

I am Russian, and I know some billionaires ("oligarchs"). They don't spend much time in Russia and don't support Putin in any way. More than anyone they realise how fucked up the regime is, which is why they stash money abroad. Putin does what he wants.

An American billionaire is a successful businessman. A Russian billionaire is an oligarch.