NEWS FLASH
Low cost carrier Wizz Air will increase frequencies on its public obligation flights from Budapest to two cities in the former Yugoslavia, which were launched earlier this summer season. Over the upcoming 2017/18 winter, Wizz will add an additional weekly frequency from the Hungarian capital to Skopje and Podgorica, for a total of three per week. The added services will come into effect from October 31. Wizz launched flights from Budapest to Skopje, Podgorica, Sarajevo and Pristina, as well as Tirana, after inking a concession agreement with the Hungarian National Ministry of Development, in order to cover these previously unserved cities from Budapest.

Comments
Especially since PSO is only legally possible as a domestic service under special conditions.
Something like Croatia does when it subsidizes flights within Croatia, I think with Trade Air.
Although this is not within one country.