If you are not a user on the other accounts you will not be able to access the accounts using your credentials. However, if the client is able to authorize you once while setting up their
OAuth, you can use that authorization code along with your client credentials (client id and client secret) to generate the refresh token. You will be able to access the client's account using this set of credentials. Please check out
this guide that lists the steps how to do so using Java. Please let me know if you have further questions.