I had to get insurance on an OA visa and tried every company provided by the Australian consule. None included transit it was just health while in Thailand. I ended up using my usual travel insurance company from Australia so if you are applying for OA you will need insurance and maybe apply from where you live.
In Chiang Mai I can recommend CIDC in Nimman. I got my top teeth done and have just returned to get my bottom ones fixed. They are super up to the moment and lovely