For working in Thailand you need a work permit, first you have to change your visa category because you cannot work on a OA class visa.
If you want to start a small business you must realize that all these foreign owned businesses require an 'alien business operation permit' or foreign business licens and approval of the Foreign Business Committee. It is costly, complicated and often impossible to obtain a foreign business license, especially for any small business.
A common solution that is used by foreigners is to start a 2 million baht share capital Thai limited company with a majority Thai shareholding (therefore considered Thai and not restricted by the foreign business act). The 2 million share capital allows the limited company to apply for 1 work permit for the foreigner.
Foreigners usually use dummy or nominee Thai shareholders who have no actual vote or interest in the company. As nominee shareholders they are technically and as a matter of law considered holding the shares on behalf of the foreigner, therefore the actual owner of the shares is considered the foreigner, therefore (if investigated) the company would be deemed foreign and operating a business illegally without a foreign business license. Foreigners are under the foreign business act not allowed to use nominee shareholders, but working with real Thai shareholders or partners may not always be preferable or even secure for the foreigner as he could lose his investment or control to the Thai partners involved.
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