Yes...health insurance is still needed for the extension of the O-A visa inside Thailand.
It must be from one of the approved insurers.
Health insurance*
A certificate of health insurance issued by Thai insurance companies covering the entire period of stay in Thailand containing the details of coverage as follows:
In-house patients of not less than THB400,000/policy year, as for out-patients, not less than THB40,000/policy year.
Please check
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for more information regarding participating Thai insurance companies.
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