Oslo is 1 hour ahead of Reykjavik
Best overlap: 8:00 AM to 5:00 PM in Reykjavik is 9:00 AM to 6:00 PM in Oslo.
For the smoothest call, aim inside 8:00 AM to 5:00 PM in Reykjavik, i.e. 9:00 AM to 6:00 PM in Oslo. Oslo is 1 hour ahead of Reykjavik, but you'd hardly notice — 9 hours of shared hours makes this feel like working in neighbouring time zones. For part of the year the maths changes: Oslo observes daylight saving, Reykjavik does not, shifting the overlap by an hour.