What is the difference between using IS NULL and =NULL?

Running the following SQL Select against two different servers and getting different results:

SELECT * ;
  FROM recur_tran_test ;
  WHERE (week_end_first <= CTOD('04/26/2002') OR week_end_first=NULL) ;
  AND (week_end_last >= CTOD('04/26/2002') OR week_end_last=NULL)

Running the following SQL Select returns the same and CORRECT results on both servers:

SELECT * ;
  FROM recur_tran_test ;
  WHERE (week_end_first <= CTOD('04/26/2002') OR week_end_first IS NULL) ;
  AND (week_end_last >= CTOD('04/26/2002') OR week_end_last IS NULL)

Curious how Oracle evaluates each...? Obviously differently.

Thanks,
Will