Is 90-day reporting required for O-A visa holders if they leave Thailand within the 90 days?

90 day report, what is it? You report that you are 90 days inside Thailand. You day of arrival is day number 1 and than you count the days. You report the first time in person at your immigration office, this to get your passport details, address and TM6 correct in the database. Second or more reports can be done by internet, between 14 days and 7 days before the due date, or by mail 15 days before due date or in person (Autohorized person) between 15 days before till 7 days after due date. if you get new passport you need to report in person again to update the system,

If you do it online you can report between 14 and 7 days prior

The day you enter is day 1

Only if you are in the country for 90 days. Resets when you leave.

Leaving the country resets the 90 days. I usually report by mail because online hasn't worked since I changed my passport. It's pretty simple and cheap.