As cloudroutine mentions, the older omnisharp-server implementation is NRefactory based, whilst omnisharp-roslyn (as the name implies) has been built atop roslyn.
omnisharp-roslyn retains a bit of NRefactory (the code actions part[1]), which was called NR6Pack and has now been renamed as Refactoring Essentials [2].
Anyway, there's been a lot of recent changes in the works, as someone has already mentioned, omnisharp-rolsyn currently has an open PR [3] which will provide a plugin system to omnisharp-roslyn. Once introduced, it will mean that the various omnisharp editors/clients can pick and choose which plugins they wish to bundle with their editor. This may include F#, VB, Cake, Fake and many others.
New contributors are, as always most welcome :)
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[1] https://github.com/OmniSharp/omnisharp-roslyn/blob/f62e74324...
That is fairly new code.
It was based on NRefactory in the past which I thought was reflection-based, but I may have been wrong.