A simple answer would be "yes", you can filter out OS usage when creating reports or looking at the data in CELUS. In fact, you have several options:
COUNTER-compliant publishers must differentiate between the OS usage and usage of journals behind the paywall so this information is included in the COUNTER data directly (and CELUS can differentiate between the two)
we included DOAJ and DOAB databases into CELUS so you can also filter out journals and ebooks that are listed in the two databases of open access journals/books
if you have your own list of OS resources, you can upload the title list to CELUS (e.g. list of ISSN, or list of ISBN) and filter the data according to your title list
