Social and intellectual abilities
Dahl was the "professor type": intellectual and more introvert than others. He was a bit shy with people he did not know and felt himself to be a little socially incompetent in some settings. He was impressed by Kristen Nygaard's social abilities and activities. But at the same time he was aware of his own professional and intellectual abilities and of his skills as an amateur musician. Furthermore he was not one to give in easily in discussions. He had a vast arsenal of knowledge in many fields: music, history, geography, culture, politics, languages, etc. He also had strong opinions on various matters and enjoyed pressing his arguments if necessary. One story often told about him typically illustrates this point: a secretary who had just started work at the Norwegian Computing Center where Dahl and Nygaard were working together rushed into her colleagues' office, telling them, shocked, that two men were fighting with each other upstairs. Her colleagues calmed her down, explaining that this fighting was only Dahl and Nygaard discussing Simula.
Dahl's style of work
It was not like him to do several things at the same time. Ole-Johan Dahl had to know that he had plenty of time ahead so that it was worthwhile taking the trouble to start working. It took him a while to become concentrated and then he was completely shut off from the world around him and could work for hours. His colleagues told me that he always delved down deep into problems rather than treating them superficially. This way of working was also carried over into the garden: he did not cover large areas but did more through work in one spot.
Dahl always had great problems getting out of bed in the morning. He was convinced that his biological clock was organized in a 25-hour cycle instead of the usual 24 hours. He used to say that he needed very little sleep, except in the morning. And so he was usually late in all his doings during the day, to other people's despair. In return he was able to work for many hours at night if necessary, and students sometimes had to wait for a shorter or longer time until the professor rushed, breathless, into the auditorium to give his lecture.
Musical activities
Dahl's shyness at putting himself forward and expressing his feelings was successfully compensated for through his musical activities. He not only had a marvelous technique but also an exceptional musical intuition for the emotional contents of music, and thanks to his technical ability, he was able to express these emotions through his playing. Typically he was not one to go for solo performances to show off. His favorite was chamber music, playing together with others. Chamber music sessions became a medium for his social life: he found his best friends and his wife through music as well as a social milieu which he highly enjoyed and appreciated and a setting where he felt he could communicate verbally without hindrance. These friendships lasted until his death. During the last stage of his cancer, his music friend often visited him and played for him to his great delight, supporting him in his distress.
As for the relation between music and science, he had an exceptional cognitive-intellectual grasp of written music. He could play almost all kinds of music at sight, without practicing beforehand. He could play unfamiliar chamber music of the most difficult sort, and if one of the other players lost their place, he would play their part in addition to his own until they found the right bar again, even if they were playing in a different clef. It seemed as though he did not read the notes one by one, rather "reading" the whole page and getting hold of the whole musical picture in one cognitive grasp. He perceived music both as structure and as an ongoing process.
As a member of an international club for amateurs players, he organized several tours through Europe together with his wife (who plays the cello), visiting fellow chamber musicians in other countries and making music together. to the great pleasure of everyone who thus got involved, new friends often visited him later in Norway to play chamber music. When he attended professional conferences, he always took along piano duets and usually succeeded in finding someone to join him.
Leisure activities
When he was not working or playing or busy with his family, he enjoyed relaxing on the sofa, reading or solving crosswords or doing jigsaw puzzles with thousands of pieces. As for sports, he loved watching TV when Norwegians were taking part in sports events. But for himself, after he had broken his skis several years earlier, his greatest wish for Christmas every years was: "No skis".
During the summer months he was not one to take daily walks but in the autumn he enjoyed walking in the woods picking mushrooms. He knew all about mushrooms. He also joined a bridge club with his neighbors and was a member for many years. His bridge friends visited him at home to play bridge when he lay bedridden in the last month of his life.