diff --git a/polyply/src/simple_seq_parsers.py b/polyply/src/simple_seq_parsers.py
index fb4b0968..0434fa19 100644
--- a/polyply/src/simple_seq_parsers.py
+++ b/polyply/src/simple_seq_parsers.py
@@ -51,6 +51,7 @@
                  "H": "HIS",
                  "D": "ASP",
                  "E": "GLU",
+                 "O": "HYP"
                  }
 
 class FileFormatError(Exception):