Dude, a Republic and a Oligarchy are on different ends of the government spectrum. You can't have a combination. If the nobility have the power, than it's an oligarch. If everyone has equal representation, then that's a republic. You can't have the two together, it's impossible.
You have to remember one thing about a dictator in an oligarch. He doesn't become, or stay, the dictator without the power of most of the rich. Kings weren't kings all by themselves. Without the power of the nobles, they were nothing. The same could be said for people like, Lenin, Stalin, Hilter, and any other number of dictators. A dictator is only as strong as his backers.