I would choose OSL as the route is only sold as 2 class but the 3 class A340-600 has ben turning up recently. Perhaps therefore there might be an opportunity to sit in the F seats if this happens. OSL is also likely to be a quieter ride both in terms of aircraft type and not having many other people in the cabin and if you are travelling alone there is much more chance of having a free seat next to you. You could also add in a greater chance of disruption at MXP compared to efficient OSL as the Italians do like to strike!

For me I can't see any advantage from MXP, OSL holds all the aces. Only one thing I would have way in the back of my mind is that OSL would not be as secure as MXP if TG wanted to make cuts. Depends if you are planning to travel in the near future or a long way off.