Most discussions I have read on the issue of Latin plurals in English only address words ending in -us, -um, -x and the feminine nouns ending in -a. I would like to know how to make the English plural of Latin masculine nouns ending in -o, such as "folio". I am tempted to say and write "folia" as the plural of "folio", and similarly "bifolia" as the plural of "bifolio" etc, although my Oxford Advanced Learner's only quotes "folios". I am sure I have read both versions of this plural in academic print before. What would you advise?
I would advise using the only plural form that you found listed in your dictionary (and in general, following the advice given by arnsholt in an answer to a related question: "Unless you are absolutely, completely sure you know the correct classical plural, or the classical plural is the normal plural, use the English plural").
The Online Etymology Dictionary explains that this word is derived from the ablative singular form of the Latin word folium "leaf, page." It does not come from a Latin noun that ends in "o" in the nominative singular.
That said, there are words like that (such as ratio; this is actually feminine in Latin, but it is in the third declension so it doesn't really affect how you pluralize it). Many of them have "n" in their other forms in Latin (the Latin plural of ratio is rationes). I've never heard or seen anyone use plurals like "rationes" in English. In my experience, people always use "ratios." (In Grammar of the Latin Language, by Leonhard Schmitz, I found a masculine example: pugio "knife," plural in Latin pugiones.)
The Latin word was folium, a neuter singular whose plural was folia. The ablative case was folio, and that is what the English word is borrowed from during the Late Latin period.
There is no reason not to use the regular English plural, folios. I have never seen anything else except for purely Latin formulations like folia botanica, which is clearly meant to be Latin not English.
Since folio itself is not a Latin word, there is no Latin plural. I would say that folio is Anglicized Italian. At any rate, it is English, so the plural is simply folios.