Using diagram styles
Diagram styles are OmniGraffle documents that define an appearance you can apply to an outline or a diagram. There are infinite ways to represent the same data; a diagram style helps you indicate just how you want the data to be represented.
The main situations when diagram styles come in handy are when you write an outline with OmniGraffle's outline view (using the Choose Diagram Style command in the Format menu) or when you import an OmniOutliner file (using the importing dialog). In these situations, the diagram style translates text items into visible objects. Several diagram styles come with OmniGraffle; try using the outline view with various diagram styles to see their effects.
When you apply a diagram style, the items of the outline are styles to match the diagram style's shapes, connection lines, and automatic layout settings. Items at each level of the outline become shape objects matching objects at the same level of the diagram. If the outline has more levels than the diagram style, the deeper items use the styles of the deepest level of the diagram style.
To create your own diagram style, choose New Resource ▸ New Diagram Style from the File menu, then choose a template to begin from. Set up a one-canvas, one-layer diagram containing some interconnected objects, styled the way you want. A diagram style should be a strictly tree-like structure; this means the connection lines should create a hierarchy without doubling back on themselves. When you're ready, save it. The default save location is inside your own Application Support folder, where OmniGraffle looks for your diagram styles. Once a template is saved in that location, it becomes available in the diagram style chooser.
If you intend to import OmniOutliner files with multiple columns, you can create a diagram style with a group of shapes, rather than a single shape, to represent each item. In this case, you should put a text label on each object so that you can associate columns with them when opening the OmniOutliner file.