What happens to my Non-Immigration OA visa if I exit and return before it expires?

No .

O/A starts on the first day of arrival into Thailand ; if you have multi entry you can come and go as many times as you like in that year .

It will be stamped in your passport .

The expiry date stays the same .

You have to apply for an extension of stay before the end of the first year .

It doesn't last for more than one year !

I have that visa .

Not sure what you’re meaning by a single entry visa.

The way the OA works is that the visa is valid for one year on approval.

Whenever you enter Thailand within that first year you are given a stamp for one year or until your medical insurance is valid until whatever is the shorter. You can come and go during this first year as many times as you like.

If you were to leave and return just before th visa itself expires and you have updated medical insurance then when you arrive you’ll be given another year stamp (or until you insurance is valid), effectively making the visa usable for two years.

The second year isn’t multiple entry and as the visa itself has expired if you leave you’d need to buy a re-entry permit (single or multiple) to keep that year stamp valid.