After Switzerland, the Netherlands was the next country in which Heiller made important contacts and here he encountered a great array of old and new organs. Haarlem played a crucial role in his career as a wonderful meeting place for artists and ideas. After his success in winning the improvisation contest, Haarlem became the first international stage on which he could be permanently present and to which he attracted large numbers of course participants.

It is hard to imagine today, but before 1950 organ improvisation did not play an important role in the Netherlands. This was to change profoundly within a few years. Reminiscing in 1964 Jos de Klerk relates:

The initial spark came from Dr. Joseph Obermayr, originally from Salzburg, who had lived in Heemstede (near Haarlem) for a long time. According to his own testimony, he had “once upon a time also played the cello with the Vienna Philharmonic,” but at that stage he worked for a car manufacturer in Amsterdam. He managed to convince the gathered company that one simply had to consider the famed Christian Müller organ in the equally famous Saint Bavo church; after all, every book on Mozart stated that the master had played it. Obermayr was just “an important figure”—he loved the organ and “was keen to do something. Also behind the venture were Jos de Klerk, father of Albert de Klerk (later a famous organist), composer Henk Badings, Gustav Leonhardt, and, right from the start, a representative from Radio Netherlands. The City of Haarlem had always organized summer organ recitals, and the city organist, George Robert, was also among the founders of this new event.