Actually, I tried it and... It works!
I flashed the script, than used this guide (again):
Than it looked like the only improvement is, that the bootloader is now unlocked. Root didn't work.
I could fix that by going to Recovery, factory reset, reinstall SuperSU an reboot. My phone now is finally rooted.
I think the factory reset part is strange, but I'm so glad I now have a rooted OPO.
When you try it, please let me know if you also have to do a factory reset.