Basically just copy all the essential Oracle files to a safe location.
The essential files are the control file, database files, redo log files and the contents of the archive log directory (if you have archiving turned on).
The archive log directory is one of your initialization parameters. To determine the actual file names of the other files do the following queries.
select name from v$controlfile;
select member from v$logfile;
select name from v$datafile;
You should probably also make a copy of your init.ora file.