One way in which I've seen OKRs used effectively is as a defense against the type of middle or upper manager who is constantly coming up with new ideas or tasks. But what if we did $shiny_thing!? Sorry, that's not in our OKRs this quarter, let's have a meeting to plan for next quarter.
It acts as a defense in both directions, too, since the manager’s boss will want the manager to stick to objectives that fit with the manager’s boss’ OKRs.