I think you meant not adjectives, but adverbs. There are no adjectives with the "-o" ending in Russian. And strictly speaking, there are no endings in adverbes, the "-o" is a suffix. The suffixes can be not only "-o", they can be "-e, -y, -a", but in the case you are speaking about (it is good/bad and so on) -- yes, it's "-o". The word "большо" isn't used, instead of it used the adjective "большой" or the adverb "велико".