LTR can be applied for in or out of country. Non immigrant OA can only be applied for outside the country. While non-immigrant ‘O’ can only be applied for from within Thailand
it is when you are applying within Thailand. Or it’s a consular process outside of Thailand. But the end result is that it brings you in direct contact with immigration. And immigration holds the keys to the gate. No point getting bogged down with the terminology as the end result is the same