DECLARE
  null_tmpString  EXCEPTION;
  tmpString  VARCHAR2(20);
BEGIN
  --someProcedure sets tmpString as an OUT parameter
  someProcedure( 'asdf', 1, tmpString );

  INSERT INTO someTable (
    column1, column2
  ) VALUES (
    tmpString, 'qwer'
  );

  IF tmpString IS NULL THEN
    --raise an error to go to the exceptions section
    --in this case, we skip the update but still
    --commit the insert
    RAISE null_tmpString;
  END IF;

  UPDATE someOtherTable
  SET otherColumn1 = 5
  WHERE otherColumn2 = tmpString;

  COMMIT;

EXCEPTION
  WHEN null_tmpString THEN
    COMMIT;
  WHEN OTHERS THEN
    ROLLBACK;
END;