This is really difficult to answer, because while these terms seem well defined, they really are not. Is Putin a dictator? By some metrics and definitions, he can be labeled that. Is Obama? Again, depends on your metrics, definitions and views. Are Iranian rulers? Are rulers of Singapour dictators?
The same goes for "repress". Is it "repression" when people have broad economic freedom like in Singapour? What about (claimed) broad political freedom but very limited economic freedom like is the ideal of Western socialists? What if the "repression" is only targeted at direct political enemies, like in Putin's Russia, but not as much at the broad populace? What about Sunni Gulf monarchies where people live the life of luxury (well, did before the oil price crashed) but are extremely restricted on a personal level both socially/religiously, and politically; but aren't typically brutally oppressed on the latter dimension in good times when nobody's revolting?
Having said that, there's definitely a strong correlation between oppression and dictatorship, which can be traced to a couple of reasons:
Dictatorships frequently (though, as you can see in the laundry list above, not always) arise in resource-poor countries.
As such, repression is needed to ensure control of said limited resources, to benefit the people in power and their supporters (the ruling clans and Sunnis like in Saddam's Iraq or Alawites in Al-Assad's Syria or North Korea).
Of course, human nature being human nature, this doesn't even need to happen in resource-poor areas, see the house of Saud in KSA; or any of the empires of the past.
Dictatorships can frequently lead to poor governance (since there's no incentive to govern well at the risk of losing political power), which increases people's discontent. Political repression allows the rulers to keep the power against that discontent.