Thank you everyone for all of your very helpful replies! It's really great to have this community.
Some reactions on your reactions:
I don't see a major conflict between OKRs and Holacracy in terms of Hierarchy. It is clearly defined that the Lead Link of group sets the strategies and the Lead Links of the Sub-Circles set their strategies accordingly. Therefore, I find that the OKR process aligns neatly with the Holacracy structure.
I do see a potential conflict of "The Insanity of the What-by-When" and OKR's quarterly deadlines. However, we definitely felt a need for alignment in our organization. Currently, I think that setting quarterly objectives (especially through a bottom-up process) seems like a good middle ground between alignment and flexibility. Re-prioritizing too frequently leads to doing a lot of things a little bit instead of doing few things right.
I really like the idea of setting the KRs as metrics for the circle. This seems like a good integration of OKRs into Tactical meetings. I'm not sure yet, if and how to integrate the individual OKRs though.