I have a Non-Immigrant O multiple entry I year visa. I have seen in this page that with the O-A you can have a free extension
I have to leave in September as I understand it you can only stay 90 days, the visa is valid until 5 November, so if I come back before that date do I get 90 days again or those it expire on the 5th of November
TLDR : Answer Summary
With a Non-Immigrant O multiple entry visa, you can stay for a maximum of 90 days per entry. If you leave Thailand and re-enter before your visa expires on 5 November, you will still be granted another 90 days of stay upon re-entry.