Oracle SQL standards do not support creating indexes on views. If you need to index documents whose contents are in different tables, you can create a data storage preference using the USER_DATASTORE object. With this object, you can define a procedure that synthesizes documents from different tables at index time.
Oracle Text Reference to learn more about USER_DATASTORE
Oracle Text does support the creation of CONTEXT, CTXCAT, and CTXRULE indexes on materialized views (MVIEW).