Can a public IP address be used as Google OAuth redirect URI?

This is not currently supported. I filed a feature request and will update on progress.

Update: Essential app verification activities have continued to make support of IP address-based apps unlikely. These verification activities are necessary to provide protections against abuse of user accounts. In addition, the cost of setting up dedicated domains has been reduced significantly since this feature was requested. Please read other responses here about possible options.

You can use xip.io to work around it.

For example: '192.168.0.50.xip.io:3000' will resolve to '192.168.0.50:3000'

I ran into this issue too and so I entered a URL with a .com extension and also entered it into my /etc/hosts file. Works like a charm.

It totally sucks that my entire app now has to be developed on an apparently 'live' domain though.