Opera has its own Feed-Reader. And it seems like its not that easy to change it (at least I am too lazy to do that, and seems like other people have tried it and didnt succeed).
I found this userspace-script, but actually I dont know what exactly to do with it, it seems like it just changes the URL’s in the link-tags. That’s useful, but it’s not what I want – I want a preview before subscribing.
Since at the moment I tend to unsubscribe from feeds rather than subscribing new ones, I wont use this too often in the near future, and in the long term I will certainly use another browser rather than Opera (wer rastet der rostet, scnr).