If you want to take advantage of the OneCMS video player you will need to download an extra piece of software. 

1: Download JW FLV Media Player (http://www.longtailvideo.com/players/jw-flv-player/)
2: Upload player.swf and swfobject.js to the root of your install
3: Enjoy!

You must abide by the rules of the software, the video player is free to use IF you are running a site without ads. If your site has ads you need to buy a license.

You can use your own player if you like but some modification to the OneCMS code may be required.