Steps to get Google OpenID working with Devise and Omniauth

application.html.haml

/ Add link to sign in with Google to your layout / views:
= link_to "Sign in with Google", user_omniauth_authorize_path(:open_id, :openid_url => "https://www.google.com/accounts/o8/id")

devise.rb

# To the Devise initializer (initializer/devise.rb) before setup block:
require "openid/store/filesystem"

# and inside setup block:
config.omniauth :open_id, OpenID::Store::Filesystem.new("/tmp")

Gemfile

# Add this line to your Gemfile
gem "oa-openid", :require => "omniauth/openid"

omniauth_callbacks_controller.rb

# And create app/controllers/users/omniauth_callbacks_controller.rb:

class Users::OmniauthCallbacksController < Devise::OmniauthCallbacksController  
  def open_id
    # You need to implement the method below in your model
    @user = User.find_for_open_id(env["omniauth.auth"], current_user)
    if @user.persisted?
      flash[:notice] = I18n.t "devise.omniauth_callbacks.success", :kind => "Google"
      sign_in_and_redirect @user, :event => :authentication
    else
      session["devise.open:id_data"] = env["openid.ext1"]
      redirect_to new_user_registration_url
    end
  end
end

routes.rb

# routes file needs to have this:
devise_for :users, :controllers => { :omniauth_callbacks => "users/omniauth_callbacks" }

User.rb

# Add to inside your user model

devise :omniauthable

def self.find_for_open_id(access_token, signed_in_resource=nil)
  data = access_token['user_info']
  if user = User.find_by_email(data["email"])
    user
  else # Create a user with a stub password.
    User.create!(:email => data["email"], :password => Devise.friendly_token[0,20])
  end
end

This does not seem to be current. I've run nearly the same thing and get:
Could not find omniauth provider :open_id

from this line:
link_to I18n.t('session.sign_in'), user_omniauth_authorize_path(:open_id, :openid_url => "https://www.google.com/accounts/o8/id"), options

So mine is a bit more complicated but it doesn't matter, since :open_id is missing. I'll post an update if/ever i fix it