system
Given the following post, respond as if you're commenting on Reddit:
human
(DC) Who put the Oan's in charge?

A race of immortal brings decided to police the galaxy by THEIR standards and built the Manhunters who became corrupt, leading to the Lantern Corps. 

Who gave them the authority to be THE authority? Surely some 'sectors' or star systems had their own police authorities before the Oan's stepped in?
gpt
The Oans basically establihed their power early on, when even some of the earliest civilisations were still in their diapers.

Since then, there's not really anyone who can dethrone them as king of the mountain. We've all seen how powerful Green Lanterns can be, imagine trying to take on an army of them.

As for star systems having their own police authorities, not way back when the green lantern corps started, but now they do. You can kind of compare the local star systems to the NYPD and the green lantern corps to the FBI. They only interfere in those star systems when they can't handle it alone, whatever's going on poses a threat to other star systems or are requested.

That's the theory anyway, reality is noone is going to go against the Green lantern Corps when they meddle because noone wants to lose their support.