class MyClass
  def old_method
    'old_method OR new_method'
  end
  alias :new_method :old_method
  # NOTE-1 : 'alias' is a keyword and not a method, that is why 
  #  there is no comma (I almost everytime type a comma there !)
  # NOTE-2 : If you want to use the method kind of syntax you can use 
  #  the 'method' Module#alias_method which does the same thing
  #  but is actually a method.
  #  so it would be 'alias_method :new_method, :old_method'
end

obj = MyClass.new
#=> #<MyClass:0x00000001859f48>
obj.old_method
#=> "old_method OR new_method" 
obj.new_method
#=> "old_method OR new_method"