Is the honor loss for having an Oni come into play considered a cost?
Isawa's Fire and honor loss
A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Is the honor loss for having an Oni come into play considered a cost?
No. It is clearly printed as an effect.
On: Isawa's Fire