In this work, we propose a methodology for classifying occupations based on skill requirements provided in online job adverts. To develop the classification methodology, we apply semi-supervised machine learning techniques to a dataset of 37 million UK online job adverts collected by Burning Glass Technologies. The resulting occupational classification comprises four hierarchical layers: the first three layers relate to skill specialisation and group jobs that require similar types of skills. The fourth layer of the hierarchy is based on the offered salary and indicates skill level. The proposed classification will have the potential to enable measurement of an individual’s career progression within the same skill domain, to recommend jobs to individuals based on their skills and to mitigate occupational misclassification issues. While we provide initial results and descriptions of occupational groups in the Burning Glass data, we believe that the main contribution of this work is the methodology for grouping jobs into occupations based on skills.