Hi,
you don't need a special tablespace for OEM repository
just create a tablespace and a user.

create tablespace REPOSITORY datafile '%ORACLE_HOME/database/repository_01.dbf' size 40M
default storage (initial 64k next 64k pctincrease 0);

create user OEM identified by OEM default tablespace REPOSITORY temporary tablespace TEMP profile default account unlock;

grant connect,resource,dba to OEM;

run the repository manager and enter the new user's credentials.

enjoy ....