Olwen +att -hp worth building?

A blade Olwen is still very good. If it were me, I’d invest in a traditional bladetome set.

Sure, why not? She could always be used as a back-up blue cavalry tome. And +Atk is a great IV. With the classic Fury/Desp blade tome build, I think she can put in some good work.

I have her, unmerged with a simple life and death + dire thunder tome set and she fairs me well in AA when I need her from time to time. What have you got to lose?

Absolutly worth using in a Horse team, since Olwen good spd can utilize Blablade weapon, she can also quad with her default weapon

You were very lucky enough to acquire her with one of her best IVs.
Just build her and use her for Arena Assault.