Men at old port finding work (8)
I believe the answer is:
'work' is the definition.
(type of musical work)
'men at old port' is the wordplay.
'men' becomes 'or' (military abbreviation for Other Ranks).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'port' becomes 'rio' (Rio de Janeiro is a port).
'finding' is the link.
(Other definitions for oratorio that I've seen before include "Eg, Walton's Belshazzar's Feast", "Musical composition for voices and orchestra based on a religious text", "Large-scale choral work", "religious music", "Handel's "Messiah", for instance".)