I work in OH for a company in CO. My W2 has fed and state tax withheld but not local (box 18-20 are blank). How do I file the required RITA local tax?

Here is a link to the RITA website https://www.ritaohio.com/ 

Your local income is most likely equal to your Ohio income, unless you changed localities during the year.  In that case, you will need to prorate your income between localities.