This document via the United States Air Force College provides a concise 1 1/2 page explanation of each of the levels and how they relate to one another.

In this article, via Defence-In-Depth, Kings College London, two historians Dr Huw Davies and Dr Robert Foley, outline the relevance of the operational art and operational level of war concept to the Napoleonic Wars, the First World War, and the Second World War.

Assistant Professor Dale Eikmeier of the U.S. Command and General Staff College presents a 4-part video series on the Operational Art. The videos are around 11 minutes each and are excellent value for someone tackling the Operational Art topic for the first time.