With Oracle Database 11g Release 2, Oracle Clusterware includes the Cluster Time Synchronization Service (CTSS).
On startup, Oracle Clusterware checks for a NTP configuration, if found, CTSS goes into Observer mode. This means it will monitor the clock synchronization and report in the Oracle Clusterware alert log if it finds a problem.
If it does not find a NTP configuration, CTSS will be active. In active mode, CTSS synchronizes all the system clocks to the first node in the cluster.
The Oracle Clusterware requires the use of "-x" flag to the ntpd daemon to prevent the clock from going backwards (Linux: see /etc/sysconfig/ntpd)