Having flown SK out of ORD many times, this station has become stricter when it comes to op-ups. The flights need to be full for upgrades to be given to C. I have had a message in the booking from an SK VP where he asks agents to upgrade me to C(as a thanks to a favor). The ORD staff has still refused to upgrade to C (they upgraded me to Y Extra). It used to be a lot easier up till a year ago.

That been said, they always welcome you to upgrade on points or cash. From Y Extra to C they charge only 225 USD or 20K points. Calling the lounge is a good idea. They can call check in and let them know you plan to upgrade on points, as UA can't do anything at IAD.

At ARN it's a lot easier and the agents are really friendly. Have them put in the upgrade request at OSL and don't be surprised if you get it for free (depending on status, as an EBG member, I have the last 8/10 upgrades at ARN for free). EB Gold members and Silver get ahead of any *G members on upgrades.

Another advice: Unless you really need the EQMs, put your points on the EB account! You get 150% miles (175% with EBG) in Economy Extra. On UA you will only earn 4260 miles each way ORD-ARN-ORD and rotten 200 something miles OSL-ARN. The total points in Economy Extra r/t ORD-ARN-OSL-ARN-ORD gives me around 20K points in Economy Extra (all SK EQMs/Basic points and redeemable)

Good luck!