Same as the answer from 'Richie' but a bit more concise.
Query for user constraints only
SELECT column_name FROM all_cons_columns WHERE constraint_name = (
SELECT constraint_name FROM user_constraints
WHERE UPPER(table_name) = UPPER('tableName') AND CONSTRAINT_TYPE = 'P'
);
Query for all constraints
SELECT column_name FROM all_cons_columns WHERE constraint_name = (
SELECT constraint_name FROM all_constraints
WHERE UPPER(table_name) = UPPER('tableName') AND CONSTRAINT_TYPE = 'P'
);