There are a lot of material available on OKRs.
If you’re getting started on OKRs you can take a look at the Weekdone Learning Center.
For advanced users, there is the Weekdone Academy.
You can find examples of OKRs from OKRexamples.co
You can find OKR templates from OKRtemplates.com
You can take a look at this free ebook “Step by Step Guide to OKRs”