Why pg_dump doesn't dump the foriegn keys?

I do

pg_dump -u -s dbname > db_schema

I look at that db_schema file, I only see the primary keys but no
foreign keys.
I don't understand what is OID use for. Will -o option dump the foreign
keys for me?
Thank you!

--
Why we want to teach our babies to talk and walk,
then later we tell them "sit down!", "be quiet!" ?

Democracy is not a better way for a solution,
it is just another way to spread the blames.

--Raymond