OKRs are inane management fad. OKRs are "hard" because Objectives are vague and Key Results are vague leading to vague definitions of success and arbitrary flag planting.
Any management concept that is simultaneously so hard that no one does it right, but yet everyone is doing it is proof in point how valueless it truly is. Of course, when compensation, bonuses, and kudos are tied to OKR, people will keep doing them, as hard or as vague as they might be.
"Good OKRs tell a story about what is important, they help you inspire people to think about why they are working on what they’re working on, and I think they carry a bit of energy that comes from seeing how it all fits together in a succinct form."
That sounds more like a mission statement to me.