What is an oratorio? how does it differ from an opera?

An oratorio is a very large musical work for voices, orchestra, etc. It is usually a narrative on a religious theme, and an oratorio is usually performed without any costumes, action, or scenery. It differs from an opera because an opera is musical theater and an oratorio is strictly a concert piece. Also operas use scenes, costumes, and lots of action while oratorios don't. Finally, operas use legends or myths while oratorios use sacred and religious topics. Hope this helped!