What is the difference between an oratorio and a cantata?

The difference between a large cantata and a small oratorio is hard to determine, but normally a cantata uses smaller, more various forces, attempts to fill a smaller dramatic canvas, and admits a much wider latitude in form.

I found a very interesting response to the question, "If each part of Bach’s “oratorio” is a separate cantata, can one call it an oratorio"?

See here, it provides some needed context to my trite statement above.

http://www-cs-students.stanford.edu/~lswartz/xmas_oratorio.pdf

Language -- Oratorio English or German while a cantata is a German biblical text.

Performance House -- Cantatas were performed in Lutheran churches while oratorios were performed in concert halls called oratoriums.

Cantatas have chorales and chorale preludes while oratorios do not.

But the difference is whether or not the piece has a definite plot, or a guiding storyline beyond the texts chosen (e.g. "Jephtah", "Elijah" or "The Messiah"). Usually oratorios focus on biblical subjects, which is why Brahms' "Rinaldo" and Bach's "Birthday" and "Coffee" cantatas are not considered to be oratorios. But in fact Bach's Passions are oratorios. Another difference is that usually (but not always) an Oratorio is structured in several acts like an opera, while a cantata has a more free structure. A cantata originally is ment to be performed during a church service (a Protestant equivalent of the Catholic Mass settings in music), the oratorio is essentially a theatrically structured sermon in music.

http://www-cs-students.stanford.edu/~lswartz/xmas_oratorio.pdf