How can I convert an ogv file to mp4?

This is an older question now but a modern FFmpeg (under Xenial Xerus and releases following this) would successfully convert an ogv file in the following manner:

ffmpeg -i input.ogv \
   -c:v libx264 -preset veryslow -crf 22 \
   -c:a aac -b:a 128k -strict -2 \
   output.mp4

and this should create an excellent file with great quality playback!

Extra Tweaks:

Some tweaks to these settings are more than possible. Here are some of my suggestions:

1. If you wanted to get a better quality video output decrease the crf setting to something like: -crf 18. Bear in mind that file size increases as the quality setting is lowered.
2. Consider adding -movflags +faststart to the command line if you wish to present your video via simple HTTP(S) progressive download.
3. Newer versions of FFmpeg (i.e. released after December 5th 2015) will not need the -strict -2 option but it is still needed for Xenial Xerus...

Arista Transcoder

or from command line

sudo apt-get install arista -y

You can use this software, that I always use and I think it's really good. To convert an ogv file to mp4 you should choose any Sony device.