You can edit any report to see what it does.
- The OEM report lists machines whose OS serial is: Like '%-OEM-%'
- The non-OEM report lists machines whose OS serial is: Not Like '%-OEM-%'
If the "OEM" part of your serials is not surrounded by hyphens, then this would explain why the reports "don't work". You can hit the edit button next to the reports under Reports/View All Reports and remove the hyphens from the Criteria column to resolve the issue. E.g. change this...
Not Like '%-OEM-%'
... to this.
Not Like '%OEM%'
P.S.: We edited/deleted some of our previous responses to make this conversation less confusing. Your original question was somewhat unclear to us.