The invention relates to a process for the preparation of a fermentation product from ligno-cellulosic material, comprising the following steps: a) optionally pre-treatment of the ligno-cellulosic material; b) optionally washing of the optionally pre-treated ligno-cellulosic material; c) enzymatic hydrolysis of the optionally washed and/or optionally pre-treated ligno-cellulosic material using an enzyme composition comprising at least two cellulase and whereby the enzyme composition at least comprises GH61; d) fermentation of the hydrolysed ligno-cellulosic material to produce a fermentation product; and e) optionally recovery of a fermentation product; wherein before and/or during the enzymatic hydrolysis oxygen is added to the ligno-cellulosic material.