User blog comment:Gary299/Dx.1 / NEW's OVA Discussion/@comment-16281343-20150310172248/@comment-16281343-20150311185114

I don't know the specifics yet.

But with all that is happening, my speculation would be it is before Volume 19 (as majority of the events in Volume 19 happened in just a matter of days including the fight preparation and fight proper). Ravel participating against Rias and Issei, I'm not sure but my bet is no.