When referring to the country, it really doesn't matter if you pronounce "Lao" or "Laos", but technically speaking if it is written on a formal document the name "Lao PDR", which means Lao People's Democratic Republic, is the appropriate form.
Lao people call ourselves "Lao", the language is "Lao" and we always pronounced the country as "Lao". The word "Laos" (La-ous) was actually misspelt by the French during their Colonization of Laos in the 1950's. The Luang Prabang accent also contributed to the misunderstanding by French officials.
The official name of the country in English language was "Kingdom of Lao" but it was mistranslated into French language as "Royaume Du Laos", so we ended up getting stuck with the spelling of "Laos". Perhaps these silly buggers had a few too many drinks.
Since the Pathet Lao government gained independence from France in 1975, Laos was changed to Lao People’s Democratic Republic (Lao PDR), also known as "Sathalanalat Paxathipatai Paxaxon Lao" which is currently the official name of the country.
Anyway that's just abit of interesting information about where the letter "s" in Laos originated from. At the end of the day there isn't really any difference if you say "Lao" or "Laos" because people will understand that it's related to the same country, the latter is just a French word.