Connected to Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 
SQL> create table dropme (id number);
Table created

SQL> set transaction read only;
Transaction set
SQL> insert into dropme values (1);
insert into dropme values (1)
ORA-01456: may not perform insert/delete/update operation inside a READ ONLY transaction

SQL> insert into dropme values (1);
1 row inserted
SQL> set transaction read only;
set transaction read only
ORA-01453: SET TRANSACTION must be first statement of transaction

SQL> savepoint before_insert;
Savepoint created
SQL> set transaction read only;
Transaction set
SQL> insert into dropme values (1);
1 row inserted

Connected to Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 
SQL> begin
  2      commit;
  3      savepoint new_savepoint;
  4      set transaction read only;
  5      insert into dropme values (1);
  6  end;
  7  /
PL/SQL procedure successfully completed

    if dbms_transaction.local_transaction_id(false) is not null then
        raise_application_error(-20000, 'ORA-01453 workaround: SET TRANSACTION must be first statement of transaction');
    end if;
    set transaction read only;