You haven't said which version so I've assumed 9.2, you should be able to rebuild the index by using

Code:
ALTER INDEX SYS.i_OBJ2 REBUILD;
if not

Code:
CREATE UNIQUE INDEX SYS.I_OBJ2 ON 
  "SYS".OBJ$(OWNER#, NAME, NAMESPACE, REMOTEOWNER, LINKNAME, SUBNAME) 
  TABLESPACE SYSTEM PCTFREE 10  STORAGE(INITIAL 16384 NEXT 106496 PCTINCREASE 0 );
HTH