An ATO is a “Approved Training Organization”. You will need the services of an ATO in order to receive training for the issue, revalidation or renewal of an instrument rating.
Not all flight schools offer this kind of training and course, but here are some:
Oslo Flight Academy, Oslo
Linköpings Flygklubb – Linköping – http://www.lfk.se/
South Sweden Flight Academy – Malmö/Sturup – https://www.southsweden.se/