builder.rb

require 'nokogiri'

xml = Nokogiri::XML::Builder.new(:encoding => 'UTF-8') do |xml|
  xml.feed(
    :xmlns      => 'http://www.w3.org/2005/Atom',
    'xmlns:osa' => 'http://www.onespot.com/Atom/Extensions/1.0'
  ) do

    xml.title 'feed title', :type => "html"

    xml.entry do |ctx|
      ###
      # default namespace
      xml.title 'entry title', :type => "html"

      ###
      # "osa" namespace
      xml[:osa].score(0.50)

      ###
      # All of these tags belong to the "onespot" namespace
      ctx.with_namespace(:osa) do
        xml.foo(0.50)
        xml.bar(0.50)

        ###
        # Including these tags
        xml.baz do
          xml.yay
        end
      end
    end
  end
end.to_xml