OmniGraffle 5 has a great many keyboard shortcuts for various menu entries and commands, so many that we made a handy document accessible from the Help Menu to umm, help out in learning these various commands.
To take it one step further, Nick Finck at Blue Flavor has compressed this lengthy document to tabloid size so that it can be printed out and referenced without covering up what you're working on in OmniGraffle, as that could become quite frustrating indeed.
We've hoisted that same document up on our OmniGraffle Extras page, and here's a handy direct link as well.
Thanks much Nick, and cheers!