Adding an OR operator?

I need to make a change to a report. Instead of a patient having only a diagnostic mammogram as an initial exam. I need to update this formula to  diagnostic mammogram or US Breast as initial. What are your thoughts? Do I need to insert an 'OR' operator in there somewhere?

Thanks

Matt

=If (Pos(Previous([Order Catalog Short Description]); "Diagnostic") <> 0 And Pos([Order Catalog Short Description]; "Diagnostic") = 0)

                  And (Previous([MRN- Organization]) = [MRN- Organization]) Then

   DaysBetween(Previous([Order Date & Time]);[Order Date & Time])

Else

   ""