Need a formula to count unique values from one column based off of another column

Hello!  This question is for a web-based Excel which I understand does not have all of the same features as a desktop Excel.  

I need to create a formula that counts unique values in one column (say, column O) based off of values in another column (say, column B).  I currently have a working countif function as =countif($O:$O), A3), where A3 is the value to look for (in this case).  Column O will have repeating values such as Waiting, Pending, Approved.  Column B will have repeating values such as Customer A, Customer B, Customer C.  As an example, I want to find out how many customers have a status of Waiting but I only want that status to be counted once per customer.  So if Customer A has 5 rows with a status of Waiting, I only want it to be counted once.  Thanks in advance for any help you can provide.