Can I stay in Thailand with a valid 90-day stamp after my Non-O visa expires?

Your 1-year multiple entry Non-O visa will expire in mid-September. However, since you received a new 90-day entry stamp in August, you are allowed to stay in Thailand until the end of October. It's important to note that your stay is based on the stamp, not the expiration of your visa, so you can remain in the country until the date indicated on the stamp, even if it exceeds the validity of your Non-O visa.