create table T (
    A  char(1),
    B  char(1),
    C  computed by (A || ' ' || B)
);

create index IDX_T_C on T computed by (lower( C ));

insert into T ( A, B ) values ( 'A', 'B' );
insert into T ( A, B ) values ( 'C', 'D' );
insert into T ( A, B ) values ( 'E', 'F' );

commit;

SQL> select * from T where lower( C ) = 'a b';

A      B      C
====== ====== ======
A      B      A B
SQL>

SQL> select * from T where lower( C ) = 'a b';
SQL>

alter index IDX_T_C active;