I have found that Crystal, at times, has issues with NULL values. I too have used Crystal with Oracle, and encountered peculiar effects when testing against null, even using the report options to convert nulls, and using isnull.

What I have found the best is to edit the sql crystal passes to the db. Using nvl(FIELD,'VALUE'), one can convert the null values to a "testable" value, which passes muster with Crystal.

You may want to create a View in Oracle which will pull the fields you need, using nvl to convert out the nulls, and have crystal pull from this View. This has led to success for a number of reports for me.

HTH.
R.