diff --git a/opportunity.py b/opportunity.py
index 18d557b8fb..601d8e7143 100644
--- a/opportunity.py
+++ b/opportunity.py
@@ -5,6 +5,7 @@ import datetime
 from itertools import groupby
 
 from sql import Null
+from sql.functions import CharLength
 
 from trytond.i18n import gettext
 from trytond.ir.attachment import AttachmentCopyMixin
@@ -236,6 +237,11 @@ class SaleOpportunity(
                     },
                 })
 
+    @classmethod
+    def order_number(cls, tables):
+        table, _ = tables[None]
+        return [CharLength(table.number), table.number]
+
     @staticmethod
     def default_state():
         return 'lead'