Steffen passed in Johannesburg on June 27.
Awarded Mining Journal's Lifetime Achievement Award in 2010 in recognition of his technical contribution to the international mining industry, Steffen served as president of the South African Institution of Mining and Metallurgy from 1989 to 1990, and received the Brigadier Stokes Memorial Award in 1995 and SAICE Geotechnical Gold Medal in 2001.
Born in 1940 in Swaziland, Steffen spent his youth and completed his schooling there before leaving for Johannesburg in 1956 to study civil engineering at Wits University.
He graduated with bachelor's and master's degrees in 1961 and 1963, respectively.
He spent seven years working for Nchanga Consolidated Copper Mines in Zambia, initially in a geotechnical role focused mainly on pit slope stability and later transferring to production, where he moved successively through the roles of shift boss, mine captain, and assistant openpit manager production and planning, before becoming openpit manager.
He left Nchanga in 1969 to take up an appointment as senior lecturer in soil mechanics and foundation engineering at Wits, where he worked with mentor,professor Jere Jennings.
Steffen remained at the university until late 1973, conducting research on rock slope stability that culminated in a PhD thesis, and also consulting for civil and mining clients.
Also in 1973, Andy Robertson persuaded Steffen and Hendrik Kirsten to join him in forming a new consulting partnership, Steffen, Robertson and Kirsten.
SRK was born in 1974.
Steffen's reputation as a consultant grew as the SRK brand went from strength to strength. Today the SRK organisation is a global group of practices owned by its leaders and employees.
A statement from SRK said: "Oskar was very much the statesman and spiritual leader in the new practice, and his personal values had an outsize influence on the culture that evolved.
"He firmly believed that ownership opportunities for strong contributors were essential in securing their long-term commitment to the business, leading to increasingly broad-based ownership in SRK.
"His strategic approach also promoted SRK's expansion into a wide range of engineering and scientific disciplines."
Steffen is survived by his wife of 53 years, Marge, his daughters Helen, Linda, and Heidi and their families, and his niece Ilse and nephew Oskar junior, who were largely raised by Oskar and Marge after their mother's death.