"In a query that performs outer joins of more than two pairs of tables, a single table can be the NULL-generated table for only one other table....
In certain situations you have to use union and subquerys instead outer-joins because using them in the way that you need you break the rule described above in the quote..

I don't know the logic of your table's structure , so maybe this query don't give the results that you expect. But you will note get an ORA-01417 error.

SELECT COUNT(*) FROM
PILOT.PERSON P
,PILOT.MEMBERSHIP M
,PILOT.HOUSEHOLD H
,PILOT.COMMUNICATION C
,PILOT.BASE G
WHERE M.PRN(+) = P.PRN
AND M.HRN(+) = H.HRN
AND M.MEMBERSHIP_NUMBER(+) = C.MEMBERSHIP_NUMBER
AND H.POSTCODE (+) = G.POSTCODE