If you don't leave the airport then you do not need a visa.
If you plan to leave the airport you will need to apply for an eVisa from the Turkish government from : evisa.gov.tr as they no longer provide visas upon arrival.
I worked for a couple of months in an office at the site of the former Oslo airport, starting in a snowy foggy early March. I don't remember seeing any evidence that anyone was changing their shoes. I certainly wasn't.
Normally, in mainland Norway (Svalbard is different) you would not be expected to change shoes at the door of a normal office. It would be at your discretion; wear what makes you comfortable.
In a private home however, at least in an informal setting, you would very often take off your outdoor shoes and either walk in your socks or wear clean indoor shoes.