OVD logs are really helpful, especially when you are debugging any issue that includes OVD.
The incoming connection to OVD server gets recorded in access.log file. $ORACLE_INSTANCE/diagnostics/logs/OVD/[ovd1]/access.log
access.log lists client accessing OVD and action requested (LDAPBIND, LDAPSEARCH, LDAPADD) which is really helpful while investigating issues.
a) Log messages are written to the access.log file only when logging is set to NOTIFICATION:1 (INFO) level.
b) You can increase the log level to ERROR:1 (SEVERE) or WARNING:1 (WARNING) to disable information from being written to the access.log file. By default access.log in OVD 11g is blank and is not updated as logger name com.octetstring.accesslog is set to WARNING:1
c) Important thing to note is that when setting log levels for Oracle Virtual Directory using Oracle Enterprise Manager Fusion Middleware Control, the following log levels do not apply to and have no effect on Oracle Virtual Directory:
NOTIFICATION: 16 (CONFIG)
TRACE: 16 (FINER)
d) OVD logging can be configured either by Oracle Enterprise Manager Fusion Middleware Control or by wlst commands. Here is the easy to do it via FMW em control.