After having read page up and page down on the internet concering the problems with OpenOffice (Impress) and videos inside presentations, I've not discovered a solution to the problem. It's not as "elegant" as in Powerpoint, but it's working and is perfectly acceptable for now.
What I do is this:
First, install Xine and its dependencies.
Secondly, instead of inserting the video itself into an Impress presentation I simply insert a textbox with the desiered text, highlight the text and inserting a hyperlink into it, pointing to the selected video. This way Xine will pop up (without taking Impress out of presentation mode!) and show the video.
After the video has finished I simply close the video window and continue my presentation. Impress is never out of presentation mode during these videos. This is a great feature with OpenOffice as opposed to Powerpoint which jumps out of presentation mode if external programs are started.
I used the way of showing videos last week when I held presentations for about 600 students in 5 different schools in Norway, and it worked like a charm!!