After upgrading to the latest OPatch version. I managed to install the PSU1 successfully.
There was however 1 issue:
After completing the opatch apply, you need to run the post_deploy_scripts.sql.
This script ends with a message saying the SYSMAN schema has one INVALID object. After querying the repository it seems that Package MGMT_JOB_PARAM_RECORD is invalid.
Metalink Note 842747.1 explains how to solve this problem.
After this all objects in the SYSMAN schema are VALID.
Please read the README.txt for instructions to install this Patch.