diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 0d98b74..f2fe447 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -276,7 +276,7 @@
 try:
     UnsupportedOperation = io.UnsupportedOperation
 except AttributeError:
-    class UnsupportedOperation(ValueError, OSError):
+    class UnsupportedOperation(OSError, ValueError):
         pass
 
 
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index 65c955a..e9f22af 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -654,7 +654,7 @@
     /* UnsupportedOperation inherits from ValueError and IOError */
     state->unsupported_operation = PyObject_CallFunction(
         (PyObject *)&PyType_Type, "s(OO){}",
-        "UnsupportedOperation", PyExc_ValueError, PyExc_IOError);
+        "UnsupportedOperation", PyExc_OSError, PyExc_ValueError);
     if (state->unsupported_operation == NULL)
         goto fail;
     Py_INCREF(state->unsupported_operation);