Overstay of less than 24 hours when leaving by an airport will not get you a fine but you will get a stamp that says overstay less than 24 hours fine waived. It will not affect applying for the visa, but it's recommended you don't overstay.
You can bring all the money you want untaxable as long as you don't stay at 180 days in this tax year and then the following year you can stay as long as you want.
O and OA are different, OA can only be bought in your home country, O can be bought in Thailand at immigration or your home country or another country. OA requires background check, and mandatory insurance.