http://ruby-ua.blogspot.com/2011/04/oauth-20-ruby-sinatra.html

app.rb

# -*- encoding: utf-8 -*-

require 'sinatra'
require 'oauth2'
require 'json'
require 'haml'

CLIENT_SECRET = '***'
CLIENT_ID     = '***'

def client
  OAuth2::Client.new(CLIENT_ID, CLIENT_SECRET,
                     :site          => 'https://api.vk.com/',
                     :token_url     => '/oauth/token',
                     :authorize_url => '/oauth/authorize'
                    )
end

get '/auth/vk' do
  url = client.auth_code.authorize_url(
    :redirect_uri => redirect_uri,
    :scope => ''
  )

  puts "Redirect to URL: #{url.inspect}"

  redirect url
end

get '/auth/vk/callback' do
  access_token = client.auth_code.get_token(params[:code], :redirect_uri => redirect_uri)
  access_token.options[:param_name] = 'access_token'
  access_token.options[:mode] = :query

  resp = access_token.get("/method/getProfiles", :params => {:uid=> access_token.params['user_id']}
                         ).parsed['response'].first

  haml '%h1 Привіт #{resp["first_name"]} #{resp["last_name"]}!',
    :locals => { :resp => resp }
end
def redirect_uri
  puts "Request URL: " + request.url
  uri = URI.parse(request.url)
  uri.path = '/auth/vk/callback'
  uri.query = nil
  uri.to_s
end