Setting up county tax is actually really straight forward. Have a look at our docs on tax, and it should explain how to set up your Orange County taxation properly.
Hi Kia, To answer your question, Shopify (as of today) does not allow you to define a Custom Tax at the County Level - you can only do it at State Level.
Surely there is a use case for many like you. I have raised this request to Shopify through their other forums as well. Will update here if I hear back from them.