Federal employees: This term refers to all individuals employed by the federal government across its many departments and agencies (e.g., Department of Defense, Veterans Affairs, IRS, NASA, etc.). There are millions of federal employees.
OPM personnel: These are employees who work specifically for the Office of Personnel Management (OPM), which is just one agency within the federal government. OPM is responsible for managing much of the federal civilian workforce policy, including hiring, benefits, and retirement programs. But OPM itself is a relatively small agency compared to many others.
In summary:
All OPM personnel are federal employees.
But only a tiny fraction of federal employees work for OPM.
