User blog comment:Hayashi H/Spring 2016 Event Guide/@comment-26486243-20160524084300/@comment-28545097-20160525174949

Clearing E7 easy does not fix node D's formation so, yes, Oyashio farming is really up to RNG and a prayer.

Tamachii: If you hit the little "+" sign next to Oyashio in poi statistics it will give you the reported drop rate by formation. As of this post the max is about 4.5% with the BWS II formation while the minimum is around 1% with the BWS IV form (standard caveats about poi stats data still apply obviously), so any formation seems to be able to drop Oyashio, but BWS II has a significantly higher chance.