Opening a Presentation Directly in Slide Show Mode

From NeoWiki

There is currently no way to open a presentation document directly in slide show mode. However, there is a workaround to overcome that issue:
Select the .odp or .pps file in the Finder, then double-click it. As soon as you see the grey bar in the top of the window, press quickly the F5 key (or fn-F5 on a laptop), and keep the key pressed until the slide show starts.

See also this thread [fr] which describes an extension that helps with this.