An OAuth token is a Base 64 encoded string containing a json object that has certificate signature, issuer and identification information about an application or user.
You will need an Identity Provider to use as an authentication end point to obtain a token. Azure Active directory is a common identity provider that I use. Many others exist, it will depend in your infrastructure as to which you have available.
An example authentication flow that I use commonly for REST and OData services I have built in Mendix:
https://docs.microsoft.com/en-us/azure/active-directory/develop/v2-app-types#daemons-and-server-side-apps