Oracle - Using index on unique constraint, what the deal?
If the USING INDEX clause is omitted, the index will be still created automatically with the same name as the constraint, in the same tablespace where table is created. This will not cause an error.

http://docs.oracle.com/database/121/SQLRF/clauses002.htm#SQLRF52225


  
  If you specify schema.index, then Oracle attempts to enforce the    constraint using the specified index. If Oracle cannot find the index 
  or cannot use the index to enforce the constraint, then Oracle
  returns an error.
  If you specify the create_index_statement, then Oracle attempts to    create the index and use it to enforce the constraint. If Oracle
  cannot create the index or cannot use the index to enforce the
  constraint, then Oracle returns an error.
  If you neither specify an existing index nor create a new index, then    Oracle creates the index. In this case:
  
  
  The index receives the same name as the constraint.
  If table is partitioned, then you can specify a locally or globally    partitioned index for the unique or primary key
  constraint.