Hello,

I have a non-imm 'o' visa which I have extended stay for a few years and recently I did a 30 day extension of stay visa run to Cambodia. Before this 30 days extension of stay runs out, I need to go to Malaysia for a few days. What will happen to my 30 day extension of stay upon returning to Thailand? Can I get a re-entry permit at the airport and return to Thailand?

Thanks in advance for any advice.

Yes, you can get a re-entry permit and just return.

But as it seems you have a 30 day visa exempt entry, you would just get a new 30 days when re-entering Thailand by air (as is your intention). You would not need a re-entry permit.

You don't get an extension by crossing the border and re-entering you get a new entry of some type which in your case appears to be a 30 day visa exempt entry.

You can just leave and re-enter again you do not need a re-entry permit.