Okay what I believe is happening here is a misuse of terminology :O AND the perils it causes when asking questions.
Let's take a minute and go over the difference between a VISA sticker and an entry stamp.
A visa is a sticker (usually from a thai consulate in another country which has an issue date, a valid until date and is sold either as a single or multiple entry.
The ENTRY stamp you get from a visa is what let's you stay IN thailand. The visa only allows you to get in for xxx days depending on the visa type. A visa can expire yet the entry stamp you get from the visa is still valid.
IN the instance of this O/P we have an O-A visa evidently issued in Aug 2018 with a valid until date of Aug 2019. That visa is good for unlimited entries during it's validity and each entry gets you stamped in for a WHOLE new year.
I believe the O/P did NOT extend their visa (which you can only be done at an immigration office BUT actually exited and re-entered the country just before the visa itself expired to get a whole new year's entry stamp taking them to Aug 2020.
The thing IS that O-A visa had a "built in" re-entry permit when it was sold (it was sold as multiple re-entry) BUT that re-entry feature is ONLY valid for the length of the visa (in the O/P's case until Aug 2019.
SO even though they had an ENTRY STAMP allowing them to stay IN thailand until 2020, if they wanted to leave and come back and get stamped in until that date again (wanted to keep that entry stamp 'alive' while they were out of the country) they would have had to BUY a re-entry permit before they left the country. Otherwise that entry stamp (the actual stamp that lets you stay IN thailand) canceled when you stamped out of the country.
@Ri**
correct me if I'm wrong on any of this please as far as what you did