SQL> drop table t purge;

Table dropped.

SQL>
SQL> create table t ( c1 int ,c2 int );

Table created.

SQL> alter table t add c_override int;

Table altered.

SQL> alter table t add c3 int generated always as ( nvl(c_override,c1+c2));

Table altered.

SQL>
SQL> insert into t (c1,c2) values (1,2);

1 row created.

SQL> insert into t (c1,c2,c_override) values (3,4,10);

1 row created.

SQL> select * from t;

        C1         C2 C_OVERRIDE         C3
---------- ---------- ---------- ----------
         1          2                     3
         3          4         10         10

2 rows selected.

--
-- you can hide c_override as well if you like
--

SQL>
SQL> alter table t modify c_override invisible;

Table altered.

SQL> insert into t (c1,c2,c_override) values (5,6,20);

1 row created.

SQL>
SQL> select * from t;

        C1         C2         C3
---------- ---------- ----------
         1          2          3
         3          4         10
         5          6         20

3 rows selected.

--
-- but you still need the app to know its a virtual column not a real one
--
SQL> insert into t (c1,c2,c3) values (1,2,3);
insert into t (c1,c2,c3) values (1,2,3)
                     *
ERROR at line 1:
ORA-54013: INSERT operation disallowed on virtual columns
SQL>
SQL>