diff --git a/lib/rex/order_book.rb b/lib/rex/order_book.rb
index ed839a1..0ba3363 100644
--- a/lib/rex/order_book.rb
+++ b/lib/rex/order_book.rb
@@ -10,10 +10,12 @@ module Rex
       @buy_side = RBTree.new
       @order_ids = {} # order_id => order
       @current_trade_id = 0
+      @current_order_id = 0
     end
 
     def add_order(order)
       side = side_for_order(order)
+      order.id = next_order_id
 
       side[order.price] ||= Limit.new(order.price)
       side[order.price].add_order(order)
@@ -66,6 +68,10 @@ module Rex
 
     attr_reader :order_ids, :buy_side, :sell_side
 
+    def next_order_id
+      @current_order_id += 1
+    end
+
     def side_for_order(order)
       if order.is_buy
         buy_side
diff --git a/spec/order_book_spec.rb b/spec/order_book_spec.rb
index 5672c53..a8c4a47 100644
--- a/spec/order_book_spec.rb
+++ b/spec/order_book_spec.rb
@@ -4,7 +4,8 @@ RSpec.describe Rex::OrderBook do
   let(:instance) { described_class.new }
 
   describe "#add_order" do
-    let(:order_a) { build(:order, is_buy: true, price: 100) }
+    let(:order_a) { build(:order, id: nil, is_buy: true, price: 100) }
+    let(:order_b) { build(:order, id: nil, is_buy: true, price: 101) }
 
     it "adds the order to the order book" do
       instance.add_order(order_a)
@@ -12,6 +13,11 @@ RSpec.describe Rex::OrderBook do
       expect(instance.best_buy_price).to eq(100)
     end
 
+    it "assigns an order id" do
+      expect { instance.add_order(order_a) }.to change(order_a, :id).from(nil).to(1)
+      expect { instance.add_order(order_b) }.to change(order_b, :id).from(nil).to(2)
+    end
+
     context "with multiple orders at the same price" do
       let(:order_b) { build(:order, is_buy: true, price: 100) }