SQL & PL/SQL :: Result Order In 2 Table Joins Without Order By

I have below tables,

describe rpthead

Name Null Type
--------------------------- -------- -------------
RPTNO NOT NULL NUMBER
RPTDATE NOT NULL DATE
RPTD_BY NOT NULL VARCHAR2(25)
PRODUCT_ID NOT NULL NUMBER

describe rptbody

Name Null Type
------------- -------- -------------
RPTNO NOT NULL NUMBER
LINENO NOT NULL NUMBER
COMMENTS VARCHAR2(240)
UPD_DATE DATE

The fact is that we store some header in RPTHEAD and store real data in RPTBODY, the question is that if I use below SQL to query all data for a 'PRODUCT_ID'.

SELECT t0.LINENO, t0.COMMENTS, t0.RPTNO, t0.UPD_DATE
FROM RPTBODY t0 , RPTHEAD rpthead
WHERE
(
t0.RPTNO = rpthead.RPTNO
AND
t0.UPD_DATE>=to_date('1970/01/01 00:00:00','YYYY/MM/DD hh24:mi:ss')
AND
rpthead.PRODUCT_ID IN ('4647')
)

I do not want to have 'ORDER by' clause since data set is too large, the sorting takes long time, is there any way to get the result rows in the order sorted by RPTNO? We have the index for RPTNO on RPTBODY.