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If you turn on the dependencies column in your view (View->View Options or cmd-J) you can type in the dependencies you want by typing the task id and then a two-letter code for the dependencies type. If you put "type a dependency code" into the search box in the OmniPlan help command, you'll get a link to the page that shows examples.

If you simply want to put finish->start dependencies on a long list of items which are entered in order, type them all in, select the whole group and click the Connection button to connect them all. Yes, it uses the mouse, but not much :-)