There are a lot of material available on OKRs.
If you’re getting started on OKRs you can take a look at a ebook by Alexander Maasik “Step by Step Guide to OKRs”
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