We will be visiting in September and just realized the Oslo marathon is on the day before we head back home. (Marathon is Sept. 21). We know there will be thousands of runners and spectators, but we wondered what effect this will have on attractions, museums, stores, etc. being open and if public transport routes will be changed. Other than the race routes and attendance in years past I can't find much other information on what to expect. We are thinking we may try to find something to do slightly outside the city for at least the first half of the day.
Cannot say something to that - just the tip from my side not to head on to Berlin the following weekend - marathon with over 44,000 runners :-) Even if Kipchoge is going to the next artificial under 2 hrs record in London 1-2 weeks after that.
Have a good time in Oslo.
My bet is that road closures and public transit detours will be announced a week or so before the actual race. At least that's what I've noticed in the US. I personally like the festive atmosphere that surrounds a big race like a major city marathon. It's a lot of fun and might turn out to be one of those "local experiences" that you fondly remember after your trip.