In manufacturing, kitting is the process of grouping separate items together to be supplied as one unit to the assembly line. This is a key logistic task, which is usually performed manually by human operators. However, picking objects from the warehouse implies a great repetitiveness in arm motion. Moreover, the weight and position of items may increase the physical strain and induce the development of work-related musculoskeletal disorders. The inclusion of a collaborative robot in the process may help to reduce the operator's effort and increase productivity. This paper introduces an online scheduling algorithm to guide the picking operations of the human and the robot. The proposed approach has been experimentally evaluated and compared with an offline scheduler, as well as with the baseline case of manual kitting.