Istanbul is 1 hour ahead of Oslo
Best overlap: 8:00 AM to 5:00 PM in Oslo is 9:00 AM to 6:00 PM in Istanbul.
For the smoothest call, aim inside 8:00 AM to 5:00 PM in Oslo, i.e. 9:00 AM to 6:00 PM in Istanbul. Istanbul is 1 hour ahead of Oslo, but with 9 hours of common ground you can hold morning or afternoon meetings without anyone working odd hours. Seasonal note: Oslo uses daylight saving and Istanbul doesn't, so double-check the times in your own months — they can move by an hour.