Most of the least corrupt countries in the world are also in the OECD; but which non-OECD countries are the least corrupt?
According to Transparency International listings for 2019
You can find a list of OECD countries here: https://en.wikipedia.org/wiki/OECD#Current_members
The CPI generally defines corruption as "the misuse of public power for private benefit". The CPI gives countries a mark out of 100, where 100 is "very clean" and 0 is "hightly corrupt".