Oauth2 flow without redirect_uri

Have you looked at this documentation? https://developers.google.com/accounts/docs/OAuth2InstalledApp#choosingredirecturi

Choosing a redirect URI

When you create a client ID in the Google Developers Console, two redirect_uris are created for you: urn:ietf:wg:oauth:2.0:oob and http://localhost. The value your application uses determines how the authorization code is returned to your application.

http://localhost

This value signals to the Google Authorization Server that the authorization code should be returned as a query string parameter to the web server on the client. You may specify a port number without changing the Google Developers Console configuration. To receive the authorization code using this URL, your application must be listening on the local web server. This is possible on many, but not all, platforms. If your platform supports it, this is the recommended mechanism for obtaining the authorization code.

TL;DR How do you use OAuth2 to authenticate users between my client and my server without redirection?

You can't. OAuth requires that the user is directed to an authorization (and possibly login) screen, and then redirected back to your app.

The redirect_uri can be a URL with a custom URL scheme for which the client registered a handler. This is described here: What's a redirect URI? how does it apply to iOS app for OAuth2.0?. It is not so much about "redirecting" it is about a callback endpoint to your app.