How Do I Filter a Group of Rows based on one Column's Value in TSQL?
Do you really need the GROUP BY, are those rows not distinct?

You can add a conditional CASE to find rows with 'Overtime Pay'

WITH RESULTS AS
(
SELECT 
      [FullName]
  ,[PayHours]
  ,[SSN]
  ,[EffectiveDate]
  ,[PayDescription]
  ,[OrderNumber]
  ,[WeekStart]
  ,[WeekEnd]
  ,SUM(PayHours)
   OVER(Partition BY FullName,WeekStart) AS TotalHours
  ,MAX(CASE WHEN PayDescription ='Overtime Pay' THEN 1 ELSE 0 END) 
   OVER(Partition BY FullName,WeekStart) AS OvertimePayFlag


  FROM [viewReportPotentialOverTime]

  WHERE EffectiveDate BETWEEN '01/01/2015 00:00:00' AND '10/23/2015 23:59:59' 

   -- really needed?
  Group By FullName,SSN,WeekStart, WeekEnd, EffectiveDate, OrderNumber, PayDescription, PayHours
  )
SELECT * FROM RESULTS
WHERE TotalHours &gt; 40  -- no HAVING needed
  AND OvertimePayFlag = 1
Order By FullName, WeekStart, EffectiveDate