SQL> create table t1
  2      as
  3      select object_id, owner, rpad('*',80,'*') data
  4        from all_objects;

Table created.

SQL>
SQL> create table t2
  2      as
  3      select * from t1;

Table created.

SQL>
SQL> create index t2ix on t2 ( owner );

Index created.

SQL>
SQL> merge /*+ append */ into t2
  2      using ( select * from t1 where object_id <= 20000 ) t1
  3      on ( t1.object_id = t2.object_id )
  4      when matched then update set data = substr(t1.data,1,79), owner = lower(t1.owner);

18447 rows merged.

--
-- a query from the same session still works
--
SQL>
SQL>
SQL> select count(*) from t2;

  COUNT(*)
----------
     97859

--
-- and from a *different* session, I can still do DML on t2
--
SQL> delete from t2 where object_id > 80000;

19493 rows deleted.