On 10R2 if the Oracle binaries are installed with the RAC, one can’t use those binaries to start Oracle Instance if CRS is not running, so in case when one needs to start the instance without CRS one needs to rebuild the oracle and it’s libraries to start the instance.

To turn off RAC
# link the oracle libraries
$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk rac_off
# rebuild oracle
$ cd $ORACLE_HOME/bin
$ relink oracle

To turn on RAC
# link the oracle libraries
$ cd $ORACLE_HOME/rdbms/lib
$ make -f ins_rdbms.mk rac_on
# rebuild oracle
$ cd $ORACLE_HOME/bin
$ relink oracle