The short answer is 'it depends'.....let me explain...
A product marketer, in my opinion, is like the CEO of a product. And just like a CEO, has to do whatever it takes to make the company (in this case product) successful. Hence, she/he should be measured on what is relevant for that specific year.
PMM OKRs depend on your answer to questions such as - what is the stage of the product lifecycle? Do you own all of product marketing or a specific PMM function? What are the business goals and objectives this year? etc.
Stage of product lifecycle
If you are working on an early stage product, then perhaps you are trying to determine product-market fit, understand market needs, or launch a product this year. If you are working on a mature product, then perhaps you are trying to beat competition, optimize pricing, and gain market share. The OKRs for both situations will clearly be very different.
Product marketing function
OKRs vary by PMM function - positioning and messaging, competitive intel, sales enablement, customer insights, etc.
Business goals and objectives
If the business objective this year is geographic expansion or establishing the partner channel org, then your OKRs should be tied to these initiatives, as you are obviously going to enable, drive and support them.
In the meantime, if you have a very traditional role and just want a list. Here is a great list by PMA to get started, but remember to adopt it and change it to what matters most to your org and PMM function.