It's a expression to ease someone for the hard work. But you can't use it if he/she is your teacher or boss and he/she did the hard work particularly for you. If you use in that situation, it sounds like you are treating him/her kindly without actually saying "thank you" nor "sorry" in spite that YOU are the one who made him/her tired. 返事 means a reply. It doesn't really make sense to put it after "Otsukaresama deshita."

It's a daily common expression~ It means good work or you worked hard today~ 干得好。 良好的工作。