I know about a package that will save an AVI (standalone).
Does anyone know of something like that but saves OGV?
NReco video converter is a .NET-based library for video conversion, commercial though. StreamCoders’ Mediasuite also provide libraries for your need.
If not, you have a number of open source video converter .NET wrappers choices here, I don’t know how reliable they are. I believe you can use any of these to convert whatever you captured in Unity via this video recorder plugin called AVPro Movie Capture.
If all fails (due to your specific requirements of encoding), you can always attempt to write one as @Benproductions1 suggested starting here, when you got the time 