Grammar books and Internet sites refrain from pronouncing any abstract criterion. The idea, for example on oxforddictionaries, seems to be "-os unless in a list of exceptions (considered closed for the future)". The list seems to stem from usage by authoritative writers, thus contains words that have been in common English usage for more than a century. Other words have been added by perceived analogy, often questionable (see here for a list intended for word games).

Obviously the original rationale was to prevent a pronunciation like "-oss". For example, "heros", thus pronounced, refers to the notion as held in ancient Greece.

Whenever a dictionary says "X or Y", writers are left alone with the problem. Not all spellings are really stylistically equivalent. From the above we may guess that "-oes" generally signals a more conservative (or learned, or British, or snobbish) style, but that may well be mistaken.

The method I use (not being a native speaker) is the following: I google the spelling in question, always in quotes, together with other word I expect to appear in texts of the desired style. In this case, I googled

"hobos" song

which obtained a large majority over the "-oes" spelling. However, I cannot see that texts containing "hoboes" have a marked conservative (or learned, or British, or snobbish) tendency in their contents.