The people of New Oslo couldn't even generate electricity to warm their cities with until we came. Pretty good chance that they're using firearms that would be at home in the 1800s. Orbital bombardment to blow up their cities... what the actual fuck? Leaving aside that you would be mass murdering civilians, there is no level of military threat on New Oslo that is worth that.
The answer to this is simple. We roll them militarily or at least threaten to do so, put into place a local government that is required to follow our human rights laws like anywhere else in the HR, and then backstop them with military force. Under the aegis of that military force, we will institute compulsory coeducational schooling for children to break the brainwashing (with safeguards so we're not creating the Residential Schools), raise the overall standard of living, educate the populace, and prosecute religious nutcases who preach things that break our human rights laws. This will be a generational effort because we are going to have to literally wait for the hardcases to kick the bucket or be outnumbered by the youth before things settle down. In the interim, expect a sizeable resistance movement alongside a lot of domestic unrest.
A few other ideas:
A) Right now, the primary industry of New Oslo is probably subsistence farming. We have breadbasket worlds that are about to have a surprise once Helghan terraforming progresses. We can make agriculture no longer an industry that requires most of the population.
B) As a result, we can have a large number of people in need of work. We can fill that need by literally building new cities en masse and putting in subsidized worker cooperatives to produce things for themselves to allow the planet to work towards self sufficiency. What we can then do is carefully stagger what jobs are offered where and to whom to mix the population. It's also a lot easier to pop a panopticon and prevent rebels with Explosives or firearms from causing atrocities.
C) We need to identify and support moderate religious figures. They don't need to be as leftist as the HR is, they just need to not be utterly shitty people. If we don't do this, the people who continue to preach will be the radicals and they'll do it under wraps.
D) Education and medicine is critical. Education shifts people away from conservative badness. Control of fertility gives women power. Both of those things will permit us the break the stranglehold the nutcase have on New Oslo.
