OKR is a goal management framework that helps companies implement strategy.
Many good materials we can refer to understand OKR, for example Perdoo and OKRs.com. And, certainly, John Doerr's book, Measure What Matters, is a must-read for anyone who is interested in OKR.
Apart from treating OKR based on its definition, it is very useful to understand OKR as...
A way of working
When a company is evaluating OKR, the leadership team is essentially on the path to adopt a new way of working.
It is like an agreed-upon convention to enable related parties understand what's expected from them. And vice versa, what they can expect from others. For example, during the OKR setup and review, I know exactly what my manager and peers expected from me, terms of my commitment (KR) and accountability (scores). Also, what I can expect from them.
OKR is a way of working that everyone understands and respectfully executes. It is not too much to say that it is the responsibility of all related individuals — not management nor HR — to oversee whether this way of working is followed and respected.
Because leadership doesn't come from a title or role. It comes from responsibility.