Can I extend my Non-Immigrant O Visa based on marriage a few days early in Chiang Mai?

You can apply for the extension of your Non-Immigrant O Visa based on marriage up to 45 days before it expires, according to responses from others who have done the same in Chiang Mai. It's advisable to apply early rather than waiting until the last minute. However, it's important to note that the extension won't be issued immediately; it will undergo a 30-day consideration period before you return to have it officially stamped in your passport.