This is an example of how you might use Oj::ScHandler to replicate the :class_object feature of JSON::Ext::Parser

custom_hash_handler.rb

class CustomHashHandler < Oj::ScHandler
  attr_accessor :output

  class HandlerHash < Hash
    def method_missing(method)
      return self[method] if key?(method)
      super
    end
  end

  def hash_start()
    HandlerHash.new
  end

  def array_start()
    []
  end

  def add_value(value)
    @output = value
  end

  def hash_set(h, key, value)
    h[key.to_sym] = value
  end

  def array_append(a, value)
    a << value
  end

end