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.