OCLC Holdings Manager is a rule-based utility that analyzes MARC21 extracts and determines changes between datasets. This comparison is then used to set and unset WorldCat holdings using the OCLC WorldCat Metadata API. OHM was designed to be consortia ready, but should work just fine for standalone libraries as well.
This tool relies on a properly configured OCLC WSKey to work. You will need an OCLC symbol that has permissions to make changes on behalf of all configured symbols. OCLC support should be able to assist in proper configuration.
To begin, you will need to modify the example settings file to fit the format of your MARC21 extracts. Exclusions are configurable on entire Bib records, individual items, or both. A mapping from ILS library identifiers to OCLC symbols is used to determine holdings. Multiple ILS libraries can share a single OCLC symbol without issue, but one library cannot have multiple OCLC symbols.
The menu.py will make use of the parameters from the settings.json or settings-.json file. You then have the option to import holdings data from a MARC21 extract, compare datasets, and in the next version send changes to OCLC. There is also a test to verify that an OCLC WSKey has all appropriate permissions to set holdings for all institutions.
OHM requires Python 3.6 or higher to run, but sees vast speed improvements using PyPy3.6 or above. Please note that a 64-bit version of either is requred if your MARC21 files are larger than 4GB in size.