so when you are on a tourist visa and apply for your O it cancels the tourist visa? I am glad you are sure, lots of people are sure and wrong at the same time.
20kthb is if you come visa exempt. You are supposed to have cash with you so immigration can see you can support yourself. It is one of the things they can ask to see.
For the DTV you are showing you have access to that money and not going to be trying to find a job in Thailand.