Ok, so you've followed this article on how to do a outbound REST call to Google with OAuth

...but now what? How do you refresh your OAuth token via script to make the API calls again? You're going to need a "Refresh Token" but no one says how to get one...until now

(Assuming you are using Client ID and Client Secret for authentication)

1. Go to the Google OAuth Playground: https://developers.google.com/oauthplayground/
2. Click the Gear Icon in the upper right of the page.
3. 1. Click Use your own OAuth credentials checkbox
   
   1. Enter the OAuth Client ID and OAuth Client secret you are using in your OAuth Application Registry [oauth_entity]
      
   2. Click Close
4. In Step 1 Select & authorize APIs, there is a input field to Input your own scopes. Fill this with the your OAuth Scope from the OAuth Entity Scopes related list on your Application Registry
5. Click the Authorize APIs button.
6. You are likely to get a popup with Error 400: redirect_uri_mismatch
7. In the popups, click the account and Allow the access to the google account
8. The OAuth Playground should now move you to Step 2 Exchange authorization code for tokens with the Authorization code filled in
9. Click the Exchange authorization code for tokens button
10. The Request / Response will populate in the right pane. Save this information! In here is your refresh_token.
11. Now you can script a call to refresh your OAuth access token using tour refresh token
12. 1. //refresh_token received from previous steps var refreshToken = "1/########################-########################-########################-########################"; //Application Registry NAME var applicationRegistry = "Google"; // GlideOAuthClientRequest var clientRequest = new sn_auth.GlideOAuthClientRequest(); clientRequest.setGrantType('refresh_token'); clientRequest.setRefreshToken(refreshToken); // GlideOAuthClient var client = new sn_auth.GlideOAuthClient(); // GlideOAuthClientResponse var tokenResponse = client.requestTokenByRequest(applicationRegistry, clientRequest); // GlideOAuthToken var token = tokenResponse.getToken(); var expiresIn = token.getExpiresIn(); var accessToken = token.getAccessToken();
    2. Might be a good idea to store your refresh token in a system property 13. Once you have things working, be sure to have your google administrator remove the redirect URI authorization to the playground.

References:

Unable to refresh access token : response is “unauthorized_client”