diff --git a/radicale/app/base.py b/radicale/app/base.py
index 2132111d..28b6f262 100644
--- a/radicale/app/base.py
+++ b/radicale/app/base.py
@@ -126,7 +126,7 @@ class Access:
 
     def check(self, permission: str,
               item: Optional[types.CollectionOrItem] = None) -> bool:
-        if permission not in "rwdD":
+        if permission not in "rwdDoO":
             raise ValueError("Invalid permission argument: %r" % permission)
         if not item:
             permissions = permission + permission.upper()