are you using the Oracle TO_DATE() function to convert the data?
what is your string format for the conversion?
edit
jsut read you need to use TO_TIMESTAMP() in oracle to get fractions of seconds:
SELECT TO_TIMESTAMP('2004-09-30 23:53:48,140000000', 'YYYY-MM-DD HH24:MI:SS,FF9')
FROM dual
http://stackoverflow.com/questions/1758219/string-to-date-in-oracle-with-milliseconds
Lowell