I got a multiple entry "O" visa that is still valid for 9 month but had to get a new passport, so the old one was canceled. I still have both passports however. Can I keep using this multiple entry visa even though it's inside the old canceled passport? Or must I get a new visa?

Your multiple entry visa is still valid in your old passport. You will show both passports when entering the country to use the visa and then be stamped into the country in your new passport.

3 minutes ago, ubonjoe said:

Your multiple entry visa is still valid in your old passport. You will show both passports when entering the country to use the visa and then be stamped into the country in your new passport.

Awesome. Thank you ????

I re-entered Thailand three times already this summer on two passports: multi-entry re-entry permit and retirement extension, both in old passport. After the third re-entry, I got a new extension and re-entry permit due to expiring extension (in August) for the new passport.