Ага, и все-таки там есть "Мерлина ради".
When Hermione Granger leaves clothes for the Hogwarts house-elves in the Gryffindor common room, Ron asks her what in the name of Merlin she is doing (OP13). He uses a similar phrase when he wonders why Neville Longbottom attacks the Slytherins after they joke about mad people (OP17).