Applying an Oracle Critical Patch Update (CPU) 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 information that can be found here is the opatch_history.txt, which is an audit trail for all operations conducted using the Opatch utility.