I just realized, that the mplayer is not able to handle the picture offset correctly if a video frame has a size that is not a multiple of 16.
The videos may look strange at the borders with mplayer, but if you play them e.g. with firefox 3.5 the video is displayed correct.
This is the reason, why oggResize and oggThumb may not work correctly at the borders (as I try to trim the output for mplayer), in case a frame is set to a width or height that is not a multiple of 16. However in most cases this is not the given (e.g. planes with size 320x240, 640x480, 1024x768, 480x320 ...)
I am working on a fix.