It's better to watch it after episode 15, Rename it to Episode 15.5 so you'll not miss the flow of the story. If you watch this special episode anywhere else like at the last after 23rd episode you will feel that it is not necessary. because at the end of this special episode there's the scene of Rudeus depressed sitting on the bed when Eris and Ruijerd enters his room, that's what connects between episode 15 and 16. That is the best place for this special episode to be watched at. I renamed episode 15 to (15.0)And I renamed this special episode to (15.5 OVA)

That's my order style (: Thanks..