The problem with the mandatory OA Visa insurance is it’s minimum level and the need to have out patient coverage, which is really not required, as out patient treatment at Thai hospitals is usually quite cheap. Much better to have Non O Visa (obtainable from within Thailand) , which does not require mandatory insurance, and then arrange whatever level of cover you feel you need. A very cheap option is to just get accident cover (motorbike, car or other types of accident) for around 5,000 THB / year.