Originally posted by Dilippatel
U can user logminor utility on archived online redo log files (if your database in Archive log mode), or else if in noarchivelog mode then u should have log files (not overwritten).

Or else you can check which user connected from Network, with os-user with IP Address, and connection time in file
$ORACLE_HOME/network/log/listener.log, U can check in who connected to which Oracle user, and the using which programm (e.g. SQLPLUS, jre if using OEM ). U can get idea who was connected, but can not justify what operations (DDL, DML) made on the database using this option.
well as many suggested, logminer shows you DDL if you are using 9i which is not the case so forget about logminer