What is the last date to leave Thailand on a multiple entry O-A visa for a further 12-month stamp?

You are on a multiple entry O-A visa that is valid until August 21, 2020, but you have a stamp allowing you to stay until September 10, 2020. To receive a further 12-month stamp, the last date to leave the country would depend on the regulations regarding re-entry on this type of visa, but generally, you would need to leave before your current stay ends on September 10, 2020. Leaving by air or land typically does not affect your ability to receive the new stamp, but it's important to check specific regulations or consult with immigration for any updates.