The Greek formation describes rule by a few. Wealth can accompany it, but wealth is not the first root’s meaning.
English drew on French, Late Latin and Greek forms.
Sources
- Merriam-Webster: olig-
The prefix expresses a small number.
- Merriam-Webster: -archy
The ending denotes government and derives from Greek archein.
- Online Etymology Dictionary: archon
Also from Greek *arkhein* "to rule" and *arkhos* "leader" (see archon): monarchy, anarchy