Can I apply for a one-year extension due to marriage immediately after arriving in Thailand on a NON-IMMI-O visa?

You cannot apply for a 1-year extension for the same reason as your initial visa immediately after entering Thailand. You have to wait for 1.5 months to 2 months for the earliest application, depending on your local immigration office. Some offices, such as Bangkok, Chiang Mai, Phuket, etc., accept extension applications when your stamp remains 45 days or less, while most offices let you apply for an extension when you have 30 days or less.