I would answer this question by giving definitions. First of all, you’d need to specify what both systems represent, by saying something like
Office automation and knowledge work systems were designed to serve the information needs, precisely at the knowledge level of the organization.
This definition would be best followed by the definitions of each system, thus highlighting the difference between them.
So, what is office automation systems?
This is the system, that collects, processes, stores and transfers data or information in the electronic office communication form. It can also be considered as a computer-based information system, which is primarily used to collect, process, store and transfer different forms of communication among co-workers (individuals, groups, etc.). E-mails, word processing, desktop publishing, document image processing, telecommunication can all serve as an example of Office Automation System. To simplify the definition, I would say, OAS is the electronic systems designed for the convenience of the information exchange.
What is knowledge work systems?
Those are systems designed to serve the information needs of an organization, but they are not presented on the physical level, as OAS. The examples of those systems are CAD, CAM, Virtual Reality Systems, Virtual Reality Modeling Language, etc.