Does anyone know what "O" numbers are?

Summary:

We have two managers asking why some of their employee's have two employee numbers, an E# which is their Person Number, then an "O" number that we have never seen before,.

Content (required):

The two managers who have reported this said it's in the Team Schedule area. The "O" numbers aren't a match for the person number either, adding to the confusion. The image above is of 2 employees and how they appear to one of the managers.

Version (include the version you are using, if applicable):

21D

Code Snippet (add any code snippets that support your topic, if applicable):