I have been to all 3 branches of Bankok, all 3 branches of Kasikorn a and 2 of Siam banks in my city, NONE will open an account on an OA visa without me going to the UK embassy in Bangkok for a letter of guarantee. Thousands of people have in the past without a visa, I am aware, people still do now at random branches in random cities. Most people now use agents as its the easiest method!
I came 15 months ago on an OA visa, where i live it is impossible to open a bank account on a OA visa, maybe Bangkok it is possible if you go from branch to branch. I am aware If you use an agent no problem, 18 months ago you could open on an OA here but it changed because of money laundering. I see many other people in other cities say the same, in this and many other groups.