I am interested in oriented matroids in the context of directed graphs and optimization. Unfortunately, I know very little of the topic. Is there a book, article or a resource that serves as a good introduction to oriented matroids, especially in the context of directed graphs? It's a bonus if the resource is suitable for an (under)graduate level course and is preferably even free.
These notes written by Winfried Hochstättler for an MAA short course on matroids look like a good, short, introduction. They also contain references to several longer references on oriented matroids.