Koos started work with Oxfam Novib in 1996 in The Hague. For many years, he worked on programme implementation in South America, mostly Bolivia and Brazil. In 2003, he became the regional campaign officer for the Make Trade Fair campaign in South America.
Since 2009, he has been working on the joint Oxfam initiative to build a joint programme for smallholder investment SCALE. Currently, he is the policy adviser for improving Aim1 programmes for selected countries.
Koos has worked for the Dutch Cooperation SNV in the highlands of Peru as advisor of a peasant association with a revolving credit fund. He worked as a bilateral expert for the Dutch Government for an integrated rural development programme in the northern provinces of Burkina Faso where he settled with his family.
Koos grew up in Holland and has an MSc in Rural Sociology of Non-Western Countries from Wageningen Agricultural University.