Oratorio uses sacred text, and is typically not staged (no costumes, lights, sets, etc.).
Which of these is a shorter version of an opera or oratorio?
Oratorio
An oratorio does not have acting, scenery, or costumes as an opera does but does have arias, recitatives, chorus, and orchestra.
Libretto
They are all performed to music.
An oratorio is a large vocal and instrumental dramatic work based on a sacred subject. The main difference between an oratorio and an opera is that an oratorio has no staging or costumes.
Which of these is a shorter version of an opera or oratorio?
Oratorio
Overature
An oratorio does not have acting, scenery, or costumes as an opera does but does have arias, recitatives, chorus, and orchestra.
Handel's Messiah is an Oratorio, which is basically an opera without a stage. The Messiah is one of the few of Handel's Oratorio's without characters, which leads to the confusion expressed by many people (including yourself). The oratorio itself has a colorful place in music history; Handel's turning to the genre occured after the British public lost taste for his operas.
Yes, you could make an educated guess. Oratorio plots are often based on sacred topics. Opera plots are about murder, romance, etc... Also, the characters in an oratorio have much less interaction with each other than the characters of an opera.
Libretto
They are all performed to music.
An oratorio is just sung using solo voice or choral pieces. The opera uses solos and choral voices too, but involves stage acting as well. For example, Handel's Messiah is an Oratorio ... La Boheme is an Opera.
I believe you mean an Oratorio
An instrumental introduction to an large work, like an oratorio or opera.