Oracle Database Read Only Mode.

Opening a database in read only mode

Opening a database in read only mode enables you to 
query an open database. Read only mode guarantees nothing
is written to the datafiles and redo log files.

SQL> startup mount

ORACLE instance started.

Total System Global Area  619360256 bytes
Fixed Size                  1338280 bytes
Variable Size             398459992 bytes
Database Buffers          213909504 bytes
Redo Buffers                5652480 bytes
Database mounted.


SQL> alter database open read only;

Database altered.

SQL> select name, open_mode from v$database;

NAME      OPEN_MODE
--------- --------------------
ORCL      READ ONLY


Go back to read write mode.

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.

SQL> startup

ORACLE instance started.

Total System Global Area  619360256 bytes
Fixed Size                  1338280 bytes
Variable Size             398459992 bytes
Database Buffers          213909504 bytes
Redo Buffers                5652480 bytes
Database mounted.
Database opened.

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE