I entered Thailand on visa exempt and was told I can apply for a 60 day visa extension if I was married to a Thai before applying for a Type O visa for marriage at the Petchabun Immigration office. Is this correct? Also is it still possible to apply in country for a multiple entry Type O Visa for marriage and what are the current requirements? If offered the 60 day visa extension for marriage are you required to apply 15 days prior to applying for the Type O visa? What are the costs and requirements?

You can apply for a 60 day extension to visit your wife and then prior to the last 15 days of it apply for a non-o visa at immigration. Requirements are here: https://bangkok.immigration.go.th/wp-content/uploads/2020/10/6.pdf

Then during the last 30 days of the 90 day stay the non-o visa will allow you can apply for a one year extension of stay. You will need 400k baht in a Thai bank for 2 months on the day you apply or proof of 40k baht income.

After getting the one year extension you can get a multiple (3800 baht) or a single (1000 baht) re-entry permit to keep you extension valid if you need to travel.

Immigration does not issue multiple entry non-o visas,

Thank you very much for your help.