Olwen +SPD/-ATK or +ATK/-HP?

+Atk/-HP without question. Olwen's -Atk version is so abysmal she won't even hit 40 attack with a Blade Tome. I have 2 of them, and they're sitting on my bench because they can't kill even red units.

I think that Olwens attack is pretty low for dire thunder, but if you want to use dire thunder, +Atk is fine.

I use a nuetral IV Owlen with Blarblade fury and desperation,
With blarblade her low base attack doesnt really matter because she has the defense and health to make use of desperation and double attacking with a blade tome covers for her low attack.
She isn't too great with a blade tome at base, but with fury, desperation and buffs she matches reinhardts damage (if he doesnt have moonbow)

If you want to use dire thunder, +atk is the way to go.