Using the tool palette
Overview 
The tool palette sits in the toolbar or floats in its own window, depending on your tool palette preferences. From left to right: the Selection tool, Shape tool, Pen tool, Line tool, Text tool, Rubber Stamp tool, Style Brush tool, Magnet tool, Zoom tool, Browse tool, and Hand tool. You can use the Tool Palette preferences to set which tools are displayed in the palette; for example, the rubber stamp tool is hidden by default.
Quick access to tools
Instead of clicking, you can press the number keys to activate tools. Each key from 1 to 0 corresponds to a tool in the palette, from left to right.
You can also set up hot keys in the Tool Palette preferences.
Persistently activating a tool
By default, clicking a tool once activates it for one use; after that one use the Selection tool becomes active again. A tiny dark blue icon appears on the tool to remind you of this.
To activate a tool persistently, click it twice. The icon goes away to show that the tool will stay active until you choose a different one.
You can change this behavior in the Tool Palette preferences, such that tools are always (or never) activated persistently.
Inspecting a tool
Normally the inspectors are used to modify a selected object on the canvas. When you are inspecting a tool, though, the inspectors modify the tool itself. That is, you are changing the style of the objects the tool creates. For instance, if you make the Shape tool's fill color yellow, every object you create with it will have a yellow fill. You can always select Restore Defaults from the tool's pop-up menu to reset it.
By default, you can Option-click the Shape, Pen, Line, or Text tool to inspect it; a tiny blue icon appears to show that the tool is being inspected. You can use the Tool Palette preferences to make tools inspectable by a normal click.
While you are inspecting a tool, objects you create with the tool are not immediately selected as you create them. As soon as you stop inspecting the tool, for example by switching to the Selection tool, the objects you created become selected.
Style menus
The Shape, Pen, Line, and Text tools have style menus; these contain various styles that you can apply to the tool itself. Click the triangle in the corner of the button or click and hold on the button to open the menu.
The very top item in the menu is the last style you set.
OmniGraffle Pro includes a Favorite Styles list, to which you can add styles you intend to reuse. You can switch freely between the current style at the top of the menu and the styles in the favorites list. Select Duplicate Style to add a copy of the selected style to the favorites list. Select Delete Style to remove the selected style from the favorites list. You can also add styles to the favorites list by Control- or right-clicking an object and choosing Add to Favorites, or by holding Option while you drag an object to the tool button.
The Canvas Styles list shows the applicable styles that exist on the canvas. If you choose one, a copy of it will replace the current style.
The Stencil Styles list shows available styles from the stencil you have open. If you choose one, a copy of it will replace the current style.
Select Inspect Style to focus the inspectors on the tool itself, so that you can change its style.
Select Restore Defaults to return the tool's style to its most basic settings.