create table my (field1 varchar2( 100 ), field2 varchar2( 100 ), 
field3 varchar2( 100 ), F2 number, F3 number);
insert into my values('a','b','c', 1 , 2 );
commit;

declare
stmt_str varchar2( 1000 );
pole varchar2( 10 );
res number;
begin
  pole:='F2';
  stmt_str:='select '||pole||' from my';
  execute immediate stmt_str into res;
  DBMS_OUTPUT.PUT_LINE(res);
  
  pole:='F3';
  stmt_str:='select '||pole||' from my';
  execute immediate stmt_str into res;
  DBMS_OUTPUT.PUT_LINE(res);
end;
/
 1 
 2 

PL/SQL procedure successfully completed.