Hi IELL!,
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As Carl mentions, you cannot do this directly through OpenLP, however there is a work around.
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Create a theme with a transparent background (I think there are problems with this if you are using a Mac)
Install another application that handles the live stream VLC is a good option for this, or if you are on Windows my favourite, ScreenMonkey for more control. (I'm sure there are many other applications that you can use)
Que the video up on the second screen and then when you want the video to show choose the theme you set up and go live with an item.
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My top tip is to set the background of your desktop to black. That way if you happen to flash a bit of desktop up in between no one will ever know ;-)