table protein does not have an attribute oid? why?

Dear All:

I want to create a delete cascade in children tables. The primary key of
parent table is oid.

CREATE TABLE Link (
Protein_ID oid,
Link varchar(128)
CONSTRAINT one
REFERENCES Protein (oid)
ON DELETE CASCADE
);

Create child table is ok.

When I insert a record into this child table, it says
"ERROR: constraint one: table protein does not have an attribute oid"

Why?

Thanks.

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