"Oppure" will sound strange in most situation in a sentence. Example: "E' un bel film oppure un brutto film?", better "E' un bel film o un brutto film?". Will be better to use like a single word sentence. Example: A: "Devi venire con me assolutamente" B: "Oppure?" = "Altrimenti?"(WHat will happen if i will not do it) Another good use is at the beginning of a sentence. Example: A: "Possiamo andare al cinema" B: "Oppure possiamo andare a teatro". It's not a big difference and anyone will understand if you use one or another, if you have any dubt use just "o".

Ciao Kathrin, Hanno lo stesso significato e si usano allo stesso modo! Oppure è solo un pó più formale!

Oppure is less casual, o is more casual.

I think it is a matter of "syllabic quantities" (or "music") as in many situations in Italian. I suppose we generally try to say the most thing with the smallest -- but when the small variant is alone, we would use the larger variant (Tiziano's answer) because, otherwise, we would have nothing to stress onto. Because we need something like a stressed syllable, that makes the difference with the other part of the phrase, to put there some emotion. It's something really "Italian". A single "long word" might be used to play around, like French often repeat "du (du du du...)" :D Nevertheless, a single "o" has place in a conflictual contest, because it's direct and cold, means "I'm challenging you": -- Voglio che fai questo (=I want you to do this) -- O? (=or?) >> You don't impress me and you'd better be not so arrogant Ciao