Set a new default template

By default, OmniGraffle opens the Single Pixel Grid when you starting a new document. Switch to something you prefer by opening Preferences, looking for New Documents under the General Preferences pane, and picking another template.

If each new document you create is a bit different, Open the Resource Browser every time you create a new document.

Under General in OmniGraffle Preferences, choose "Open Resource Browser" or pick a new template when creating a new document.