An oratorio includes a choir, soloists, an instrumental ensemble, various characters and arias. However, opera is musical theatre, while oratorio is strictly a concert piece – though oratorios are sometimes staged as operas, and operas are sometimes presented in concert form. In an oratorio, the choir often plays a central role and there is generally little or no interaction between the characters and no props or costumes.
Opera tends to deal with history and mythology, including age-old devices of romance, deception, and murder, whereas the plot of an oratorio often deals with sacred topics, making it appropriate for performance in the church