When people hear "oligarch" they probably automatically assume I simply mean billionaires and other extraordinarily wealthy people, but as with most things in life, it's not that simple.
In a nutshell, while almost all oligarchs are extremely wealthy (or have access to extreme wealth), not all people with extreme wealth are oligarchs. The term oligarch is reserved for those with extreme wealth who also want to control the political process, policy levers and most other aspects of the lives of the citizenry in a top-down tyrannical and undemocratic manner. They think they know best about pretty much everything, and believe unelected technocrats who share their worldview should be empowered so that they can unilaterally make all of society's important decisions. The unwashed masses (plebs) in their minds are unnecessary distractions who must to be told what to do. Useless eaters who need to be brainwashed into worshipping the oligarch mindset, or turned into apathetic automatons incapable or unwilling to engage in critical thought. Either outcome is equally acceptable and equally encouraged.