I have not implemented oAuth yet, but I have a working setup with SAML protocol. The way to get this to work with HPE PPM is to use the generic single sign-on feature of HPE PPM. You need to configure HPE PPM with a front end web server (e.g. Apache) and implement the authorisation protocol on the Apache server. The Apache will then forward a request parameter with the authorized user's name (e.g. AUTHORIZED_USER). HPE PPM then trusts the Apache server that this user is properly authenticated.
The protocol you use in Apache to authenticate the user in Apache is invisible to HPE PPM. You may want to take a look at Apache Oltu for oAuth support.