Use a query abaixo para descobrir a Primary Key (PK) de uma tabela no Oracle:
SELECT
B.COLUMN_NAME
FROM
ALL_CONSTRAINTS A,
ALL_CONS_COLUMNS B
WHERE
A.CONSTRAINT_NAME=B.CONSTRAINT_NAME AND
A.OWNER=B.OWNER AND
A.TABLE_NAME='<TABLE_NAME>' AND
A.CONSTRAINT_TYPE='P';