The most important Oracle log is the alert log, which is found in $ORACLE_HOME/admin/$SID/bdump/alert_$SID.log .
An Oracle database performs logging on a wide array of functionality. The majority of the logs that are generated are stored under $ORACLE_HOME/admin/$SID/. Many logs are stored under this directory hierarchy, but alert is the most important. This log records all database activity, including serious problems.
The alert log is not rotated or overwritten, and can become quite large over time, especially on an active database.
Additional logs are created under the same directory hierarchy, for specific incidents. If a problem is recorded in the alert log, the other logs should be inspected for additional details.
For more information, as well as support in the maintenance of an Oracle database, contact Oracle Support or Oracle's Metalink site.