Istanbul to Oslo is about 2,425 km and usually takes around 4h in the air. The track it follows runs over the Balkans, the Black Sea, the Eastern European plains, Central Europe, and Scandinavia.
Over the Black Sea, the Balkans, the Eastern European plains, and Central Europe the air is usually calm and settled. Nearer Scandinavia the weather turns more Atlantic and the ride can be livelier for short spells, still firmly within the routine. This track threads jet-stream latitudes, so in winter you may feel a few minutes of light, choppy air high up; summer usually rides calmer.
For most of a flight this length you are at a steady cruise above the weather, where the ride is smooth; the livelier moments, if any, are short.