Word used to refer to a large group when defining a smaller one? [duplicate]

I’m trying to remember an uncommon word. It defines an exaggeration of language to include a large group of people when speaking about a smaller group. Like “All of Chicago turned out for the game last night”.

I thought it started with an O… I just can’t remember.

You might be looking for 'overstatement':

Overstatement

: the act of describing or explaining something in a way that makes it seem more important or more serious than it really is; exaggerate

(Cambridge)

'Synecdoche' doesn't start with an O, but it seems to be the best fit for your given example.

Synecdoche is a type of figurative language that uses one part to refer to the whole, or the whole to refer to the part.

In your case, a whole is being referred to a part.