Applying a Critical Patch Update or CPU patch to an Oracle database creates a log file for every patch attempt, successful or not.

The patch log can be found in:

%ORACLE_HOME%\cfgtoollogs\opatch\<patch_attempt_by_time>.log

Note that this is the path and file mentioned as part of the CPU patch installation through OPatch.

Other cool information that can be found here is the opatch_history.txt, which is an audit trail of sorts for all operations conducted using the opatch utility.