For a user to create indexes, he/she needs the same privilege as the creating tables. Just make sure he/she has the CREATE TABLE privilege. The following tutorial exercise gives you a good example on creating view privilege:

>.insqlplus /nolog

SQL> connect SYSTEM/globalguideline

SQL> GRANT CREATE TABLE TO dev;

Grant succeeded.

SQL> disconnect

SQL> CONNECT DEV/developer

SQL> CREATE INDEX ggl_index ON ggl(id);

Index created.

SQL> DROP INDEX ggl_index;

Index dropped.

SQL> CREATE INDEX ggl_index ON ggl(id);

Index created.