If you leave all the search fields blank but narrow the date range (for example, 03/01/2015 to 03/18/2015) you can see all orphan drugs that were added in that time period. Using my example, you'll see that 18 drugs have been granted orphan status so far this month and that they update the list every few days.

http://www.accessdata.fda.gov/scripts/opdlisting/oopd/