I am in Thailand and my visa isn't convertible inside the country and have to leave and come back on a tourist visa and that's how my local immigration explained it to me. The only reason I know. You won't need all the insurance requirements that are needed for the OA either. Although I have insurance already anyway. Good luck.
The OA you can only get in your home country. You can come in on a tourist visa and convert to a non O based on retirement if you meet the requirements.