Øystein worked at the Norwegian Computing Center for 4 years as project leader for a Simula machine. From 1984-88 he worked at SimTech, and from 1988-1990 as senior research scientist at ABB Corporate Research, Norway. Øystein was an independent consultant while working on his Ph.D., until he joined Ericsson Norway in 1997 to work on software methods and languages. Øystein is Associate Professor at the University of Oslo.
Øystein participates in the standardization work in ITU, first in the development of the object-oriented extension of SDL that where incorporated in the 1992 version of SDL ("SDL-92"), and later as Associate Rapporteur for MSC in ITU-T Study Group 10, that produced the 1996 version of Message Sequence Charts. He is currently Rapporteur for MSC in ITU-T, responsible for the development of MSC standard year 2000.