Good morning, can I get a re-entry permit at any border crossing when leaving the country or have to do it beforehand?

Only at airports you can get them at the border.

Get it from your immigration office before you leave.

If your visa is still valid you don't need a re-entry permit.

You have to get one before leaving the country. Best to get one at your local immigration office.

In the case that I leave the country urgently without it, what would happen when I come back?

You current visa or extension would be void if you leave without a reentry permit

Ok, thanks

We do not know what you have so it can not be answered. Do you really have a non immigrant O-A (retirement) visa issued by Thai Embassy? Is it multi entry (most are)? Is it still valid for the date you will return? If yes then you just use your visa for entry again and receive a new one year stay.

As ubonjoe/lopburi3 wrote: check the validity of your O-A (ENTER BEFORE date).

If you return before that date there is nothing to do.

(assuming that it is a multiple [M] visa as is usually the case with a Non O-A)

Edited September 2, 201411 yr by KhunBENQ