Using the feminine prefix, "la", with a noun ending in "o" doesn't sound quite correct, given that the prefix "el" would normally be used with a masculine term (I would think) regardless of country.

There are many words that break the "gender" rule in Spanish, which can drive a person crazy.

el dia
la mano
el problema

Sometimes it's because it's a shortening of the word -- for example, la foto, which is a shortened version of la fotografia. That may be why radio goes either way, but I don't know what the original word might have been.

El radio can also be "the radius" in a geometric way or a spoke, so of course context is important.

Peter

Right on Peter.

There are tons of exceptions and those are learned by rote. In the case of "radio" as we use it, the receiving box is usually called "el radio". Radio as in broadcast station is "la radio" which is a contraction of "estación radiodifusora" .

I think that they were created to confuse non native speakers....

Then again, English has its own batch of oddities too; you'd think that since the plural of foot is feet, boot would be beet... etc, etc.

Similar to the shortening of fotographia I agree that the root of "la radio" is "radiodifusion" (sp?) implying 'broadcasting'. The stations ID as 'La 95', etc. Yeah, the Spanish language has oddities just like English does but maybe not so many. And it does vary regionally.
A common one for beginners is "el agua fria'".

Yep, just like English... Different parts of world speak it differently, not just the pronunciation but the spelling and meaning of words.

There are special dictionaries that outline the differences in Spanish from country to country and even regions within a country.

When translating documents my first questions regardless of whether Sp-En or En-Sp are "what's the target country and demographics?".

re: "el agua fria"... it depends.... if it's plural it's "las aguas frias"