I am just starting the process of learning to access APIs using Python. Initially I presumed OAuth User IDs were generated or supplied by the API owner somehow. Would it actually be fabricated by myself, using alphanumerics and symbols? This is just for a personal project, I wouldn't have a need for a systematic creation of IDs. The APIs I am working with are owned by TDAmeritrade.
OAuth is has no capabilities for any type of provisioning of users or credentials.
OAuth 2.0 provides for the Delegation of Authorization

By the Resource Owner (User)
to the OAuth Client (Application)
for Access to a Resource Server (Website or Api or ???)
Creation of OAuth User IDs