SkyTeam airlines Delta/KLM/Air France have San Francisco to Copenhagen, Denmark for $429 round trip for travel departures from April to early June 2018.
Oslo, Bergen, Trondheim, Kristiansand and Stavanger are around $480 round trip. Stockholm and Gothenburg, Sweden under $500.
Similar fares are available to Oslo and a few other cities with Star Alliance airlines Lufthansa and Swiss too.
Delta Airlines San Francisco SFO – New York JFK – Copenhagen CPH – JFK – SFO