Using stencils to keep and share commonly-used objects
A stencil is a set of useful objects that you can drag into your own diagrams. To see your stencils, select Stencils from the Window menu.
Click these buttons to see the objects as they appear on the stencil or separated out with each in its own box. 
Use the Action pop-up menu to edit stencils, customize the stencils list, or create new stencils; see more about these commands below. 
Type something in the search field to find stencils whose names match, or stencils containing objects whose text or notes match. You can even search online to find stencils other people have made. See below for more about stencil searching. 
Select a stencil in the list to see its contents; select a whole folder to see the contents of all the stencils inside. 
The stencils you select appear here; drag objects from here to your document. If you have opened a stencil that isn't yet installed, click the Install button that appears in the title bar to copy it into your home folder for safekeeping. 
When you select an object, some information about it, including any notes or custom data it may have, appears here in the metadata pane. 
If you don't want a whole object, but just one or all of its styles, drag a style from the style tray to an object in your document. 
Click this button to show or hide the metadata pane. To put an object on the canvas, just drag it from the stencil and drop it wherever you like. A copy of the object is made and the original remains on the stencil, so take as many copies as you need. You can do the same sort of selection tricks that you can do on a canvas, like Command-clicking or dragging a rectangle to select multiple objects.
The stencil list is divided into several sections.
Your Stencil Library includes all of the stencils you have installed, whether they're built in to OmniGraffle or in an Application Support folder.
Recent Stencils lists the last five stencils you looked at. If you open a not-yet-installed stencil, it appears in the recent list; you can click the Install button near its title to copy it to the right location in your Home folder.
The Favorite Stencils section is fully customizable. You can drag any installed stencils you like into it. Choose New Favorite Stencil Folder from the Action pop-up menu to create a folder for organizing your favorites.
To search your stencils, type in the search field above the stencil list. OmniGraffle searches your stencils' titles, the text of the objects on each stencil, and the notes and custom data. Stencils that match appear in the special Search Results section of the stencil list. Normally, if any object on a stencil matches, then all objects on that stencil appear in the results. But if you turn on the Filter setting in the seach field's pop-up menu, then objects that don't match what you typed disappear.
To search the online OmniGraffle stencil repository, click the search field's pop-up menu and choose Graffletopia. This is Patrick Crowley's free service, where anyone can share useful stencils. When you search Graffletopia, stencils whose names or descriptions match appear in the Search Results list. Click a stencil to see a preview and a description, then click the Download button to get a copy for yourself. Open the file that appears in Finder, and then you can use or install the stencil normally. You can also visit Graffletopia from OmniGraffle's Help menu, or by going to http://graffletopia.com.
To make a new stencil, choose New Resource ▸ Stencil from the File menu. You can edit the document that appears just like you would edit a normal OmniGraffle diagram. A preview of the stencil appears in the stencil window as you work. Once the stencil looks just how you want it, choose Save from the File menu; the default location for saving is the Application Support folder inside your Library.
To edit a stencil, select it in the list and then choose Edit Stencil (or Copy and Edit Stencil, if it's built-in) from the Action pop-up menu above. Then edit and save the stencil just like a normal OmniGraffle document.