Google OAuth: App or Server?

I'd like to implement Google OAuth. I'm a bit confused whether to have it implemented in the client (mobile app) side or the server side!!

Implementing it in the client side would mean shipping the key and secret in the app (which I want to avoid). However, if I do it in the server side, how do I send back the correct response to the client after a successful callback?

Also, what are the benefits of each of the two implementations?