I think you have it backwards. "Mono" as a prefix means one, while "oligo" means...

netheril96 on May 25, 2018 | parent | context | favorite | on: Apple rejects Valve’s Steam Link game streaming ap...

I think you have it backwards. "Mono" as a prefix means one, while "oligo" means several. Therefore monopoly is a market with a single dominant player, while "oligopoly" would be a market with few but more than one dominate players.