The chorus in a Greek play provides information for the audience. This might involve summarizing action or giving new information. The chorus is a group that exists to “comment with a collective voice on the dramatic action” (enotes reference, chorus). Often the chorus can be funny.
The chorus gives information, even when they start to get annoying. However, the information is often useful exposition. Here we learn about the plague facing them.
Alas! for I bear countless woes;
disease falls upon my entire crew,(180)
and no mind’s weapon can protect me,
for the fruit of our famous land does not grow,
nor do our women emerge from their
mournful labors with offspring. (enotes pdf p. 8)
The direct interaction between the chorus and Oedipus helps characterize Oedipus. Here, he argues with the chorus:
Just as you adjured me under a curse, my lord,
so shall I speak. For neither did I kill
nor am I able to show the killer.
But it is the task of the one who sent it,
Phoebus, to say whoever has done this thing.(290)
You have spoken justly, but no man can
compel the gods when they are unwilling. (p. 11)
Thus in Oedipus Rex, the chorus becomes a character, and part of the action. This was not always the case in Greek plays, but it did add another element.