All of the relevant words are spelled with "(o)ur".
You're right to see a certain inconsistency in the lack of simplification of words spelled with "ou" before other letters. Per Wikisource, Noah Webster, one of the big influential figures in the development of distinct American spelling standards, gave the following reasons for his choice of "or" spellings in the "Orthography" section of his Compendious Dictionary of the English Language:
- Using "-or" makes spelling more consistent by bringing the spelling of "-o(u)r" words in line with the spelling of existing words that were already spelled with "-or" all the time by everybody
- The spelling "our" is inadequate because it is "neither Latin nor French, nor calculated to exhibit the English pronunciation"
- It is an "absurdity" to have -our in the spelling of words that are used as the base of words spelled with -or-, such as superiority, inferiority
The second and third justifications would apply equally well to the ending -ous: we write curious even though "ou" corresponds neither to the Latin spelling, nor the modern French spelling, nor the modern English pronunciation, and even though the derivative curiosity is spelled with just "o".
So I think the reason for not changing the spelling of -ous words to -ous is that even though -ous was not a particularly simple spelling, it was generally consistent already among words of this type. There weren't a large number of preexisting words that were already spelled with "-os" and pronounced the same as the words spelled with "-ous".
In contrast, the spelling "-or", pronounced the same as "-our", existed already for a long time before Webster's efforts to reduce the use of "-our".
Here's an article by Ben Zimmer about the gradual rise of -or spellings: "Why Americans Celebrate Labor (and not Labour) Day", September 7, 2009, Vocabulary.com Blog Word Routes