Firstly it depends on the Operating system.
Secondly, there are a number of files that make up an Oracle database. These are the:
- Control File
- Parameter File
- Online Redo logs
- Tablespace files
- Backup files, archived log files and other optional stuff
[I've community wiki'd this in case I've missed any.]
I've got a default install of Oracle XE on ubuntu linux.
My control file is /usr/lib/oracle/xe/oradata/XE/control.dbf
I've got a parameter file of /usr/lib/oracle/xe/app/oracle/product/10.2.0/server/dbs/spfileXE.ora
My online redo logs are in /usr/lib/oracle/xe/app/oracle/flash_recovery_area/XE/onlinelog
My tablespace files are in /usr/lib/oracle/xe/oradata/XE/