Firstly, welcome to FT !

Not sure if they codeshare with Croatia (OU), but if they do, then you may find ORD-SPU fares available with the following all-Star options.
LH via FRA, LX via ZRH, OS via VIE. SK also serve (via ARN) SPU on their own metal, so that's another option. Croatia serve FRA ZRH and VIE with nonstops to/from SPU at that time of year.
If LH/LX/OS do codeshare on OU, then you should be able to buy the revenue ticket ORD-SPU and also then claim your Star Reward ticket all the way through as well. Even if they don't code, they may still offer a through fare. SK should obviously offer it's own fare as it's all SK metal operated.

It's also possible UA may have a codeshare or a through fare available as well, if they have any deals with OU.

Cheers.