Thank you all. Here's what I have encountered:
Air Canada Website: sells neither IND-EWR-OSL nor ARN-EWR-IND, but sells IND-YYZ-LHR-OSL with SAS operating LHR-OSL.
United Website: sells ARN-EWR-IND but NOT IND-EWR-OSL
SAS website: cannot find either one. Probably something to do with the dates of travel?
ITA matrix shows I should be able to do IND-EWR-OSL/ARN-EWR-IND if you "call a travel agent."
Last edited by psoque; Dec 25, 2023 at 10:14 am