12 October 1931|
|Died||29 June 2002
|Awards||Turing Award (2001)
IEEE John von Neumann Medal (2002)
|Institutions||Norwegian Computing Center
University of Oslo
Ole-Johan Dahl (12 October 1931 – 29 June 2002) was a Norwegian computer scientist. Dahl was a professor of computer science at the University of Oslo and is considered to be one of the fathers of Simula and object-oriented programming along with Kristen Nygaard.  
Dahl was born in Mandal, Norway. He was the son of Finn Dahl (1898-1962) and Ingrid Othilie Kathinka Pedersen (1905–80). When he was seven, his family moved to Drammen. When he was thirteen, the whole family fled to Sweden during the Occupation of Norway by Nazi Germany. After the end of the war, Dahl studied numerical mathematics at the University of Oslo.
Dahl became a full professor at the University of Oslo in 1968 and was a gifted teacher as well as researcher. Here he worked on Hierarchical Program Structures, probably his most influential publication, which appeared co-authored with C.A.R. Hoare in the influential book Structured Programming of 1972 by Dahl, Edsger Dijkstra and Hoare, perhaps the best-known academic book concerning software in the 1970s. As his career progressed, Dahl became increasingly interested in the use of formal methods, to rigorously reason about object-orientation for example. His expertise ranged from the practical application of ideas to their formal mathematical underpinning to ensure the validity of the approach.
Dahl is widely accepted as Norway’s foremost computer scientist. With Kristen Nygaard, he produced the initial ideas for object-oriented (OO) programming in the 1960s at the Norwegian Computing Center (NR) as part of the Simula I (1961–1965) and Simula 67 (1965–1968) simulation programming languages. Dahl and Nygaard were the first to develop the concepts of class, subclass (allowing implicit information hiding), inheritance, dynamic object creation, etc., all important aspects of the OO paradigm. An object is a self-contained component (with a data structure and associated procedures or methods) in a software system. These are combined to form a complete system. The object-oriented approach is now pervasive in modern software development, including widely used imperative programming languages such as Java and C++.
He received the Turing Award for his work in 2001 (with Kristen Nygaard). He received the 2002 IEEE John von Neumann Medal (with Kristen Nygaard) and was named Commander of the Royal Norwegian Order of St. Olav in 2000. 
- Multiple index countings on the Ferranti Mercury computer / by O.-J. Dahl. Oslo: Norwegian Defence Research Establishment, 1957.
- Programmer's handbook for the Ferranti Mercury Computer, Frederic at the Norwegian Defense Research Establishment / By O.-J. Dahl, and Jan V. Garwick. – 2nd ed., Kjeller: Norwegian Defence Research Establishment, 1958.
- Automatisk kodning: et prosjekt ved Forsvarets forskningsinstitutt.
- Simscript implementation / by Vic Bell and Ole-Johan Dahl. Oslo: Norwegian Computing Center, 1963.
- Basic concepts of SIMULA: an ALGOL based simulation language / by Ole-Johan Dahl and Kristen Nygaard. Oslo: Norsk Regnesentral, [1965?].
- SIMULA: a language for programming and description of discrete event systems : introduction and user's manual. Oslo: Norsk Regnesentral, 1965.
- Discrete event simulation languages: lectures delivered at the NATO summer school, Villard-de-Lans, September 1966 / by Ole-Johan Dahl. Oslo: Norsk Regnesentral/Norwegian Computing Center, 1966.
- SIMULA: an ALGOL based simulation language / by Ole-Johan Dahl and Kristen Nygaard. Oslo: Norsk Regnesentral, 1966.
- Simula: an ALGOL-based simulation language / Ole-Johan Dahl and Kristen Nygaard. New York: Association for Computing Machinery, 1966. I: Communications of the ACM; 9(1966).
- Class and subclass declarations / Ole-Johan Dahl and Kristen Nygaard. Amsterdam: North-Holland, c1968. I: Simulation programming languages: proceedings of the IFIP working conference on simulation programming languages, Oslo, May 1967 / O.-J. Dahl, conference chairman; organized by IFIP Technical Committee 2, programming languages; edited by J.N. Buxton.
- Discrete event simulation languages / Ole-Johan Dahl. London: Academic Press, 1968. (Programming languages : NATO Advanced Study Institute / edited by G. Genuys.)
- SIMULA 67: common base language / by Ole-Johan Dahl, Bjørn Myhrhaug and Kristen Nygaard. Oslo: Norsk Regnesentral, 1968. (Publication S / Norwegian Computing Center; 2) Rev. 1970: Common base language (Publ.; 22).
- "Ole-Johan Dahl". Store norske leksikon. Retrieved May 1, 2017.
- Eirik Rossen. "Kristen Nygaard". Norsk biografisk leksikon. Retrieved May 1, 2017.
- Kristen Nygaard. "Ole-Johan Dahl". Norsk biografisk leksikon. Retrieved May 1, 2017.
- "Tribute to Ole-Johan Dahl". University of Oslo. Retrieved May 1, 2017.
- Recipients IEEE John von Neumann Medal
- "Ole-Johan Dahl". amturing.acm.org. Retrieved May 1, 2017.
- From Object-Orientation to Formal Methods: Essays in Memory of Ole-Johan Dahl, Olaf Owe, Stein Krogdahl and Tom Lyche (eds.), Springer, Lecture Notes in Computer Science, Volume 2635, 2004. ISBN 3-540-21366-X. doi:10.1007/b96089.
- Pioneer who Prepared the Ground for Road to Java, Jonathan Bowen. The Times Higher Education Supplement, 1677:34, 4 February 2005.