Do I need a re-entry permit if I hold a Non-Immigrant O multiple entry visa before leaving Thailand?

As long as you hold a multiple entry Non-Imm-O visa category, you can re-enter Thailand an unlimited number of times within the visa validity. This is the date which is noted behind the text "visa must be used by". Each entry gets you stamped in for 90 days. If you re-enter one day before the end of the visa validity, you can actually get 15 months out of your Multi Entry Non-Imm-O visa

FFS, 🙄

If you have a multiple entry visa why would you need a re-entry permit?

As long as you have a multiple entry visa, you can enter Thailand an unlimited number of times before the the date on the visa.