Você deseja usar a máscara de formato de data RR para alterar 90 para 1990:
SELECT employee_id, last_name,hire_date
FROM employees WHERE hire_date > TO_DATE('25-JUN-90','dd-mon-RR');
SELECT employee_id, last_name,hire_date
FROM employees WHERE hire_date > TO_DATE('25-JUN-90','dd-mon-RR');
