I read it multiple times already in Measure What Matters by John Doerr, that OKRs are flexible. I normally underline those things, but now I have to note it somewhere where I can find it again without physically needing the book. On page 54 he writes
Remember that an OKR can be modified or even scrapped at any point in its cycle. Sometimes the "right" key results surface weeks or months after a goal is put into play.
The TL;DR for me is the sentence after it
OKRs are inherently works in progress, not commandments chiseled in stone.
Learning and mastering OKRs takes time and practice. On page 68 Brett Kopf of Remind says:
OKRs are basically simple, but you don't master the process off the bat. Early on, we'd be off by miles in our company-level objectives, mostly on the way-too-ambitious side.
On page 75 David from Nuna confirms that:
You're not going to get the system just right the first time around. It's not going to be perfect the second or the third time, either. But don't get discouraged. Persevere. You need to adapt it to make it your own.