A Brand New Olwen Idea.

Nice, i like it. Just be sure to give her hone/fortify buffs and she is awesome.

Oops, double post

That's disgusting, but awesome
Add horse buffs, maybe a Distant Def seal, and you have yourself an absolute monster

I have a +res Olwen and I would do this in a heartbeat if I had a Morgan I was willing to fodder. Sadly I only have one and even though I hate her IVs (+spd -res, pretty much the exact opposite of what I would’ve wanted), I can’t bring myself to get rid of my only copy.

Sure, she can tank, but 41 atk is literally terrible.

I've thought about this Olwen before, but the reason I didn't invest in her is because she's going to need to use All of her stats.
Blarserpent demands that her HP/Def/Res all stay as good as they can be, and speed directly affects her tanking capabilities.
While technically you can sacrifice her attack to make her defenses stronger, not only is it immensely difficult to fine-tune her IVs as a 5*-exclusive unit, her attack is also terrible anyway, why would you make it worse with a superbane?

Overall, I think there's great potential, especially considering she has really good res and speed, but it falls short because it requires her to use all her stats instead of maximizing her strengths and minimizing her weaknesses. While many heroes can afford to utilize all their stats, and have the BST to do so, Olwen is hardly one of them.

I'm very interested in trying out something like this however. Fury and Renewal are very budget-y skills and I think for the purposes of tanking, there are better skills Olwen can use (for example, some Bond skill in A like Spd/Def or Def/Res, maybe even a stance like Mirror to make her exclusively enemy phase, etc...)
For B-slot, you can even experiment with Dull Ranged/Close to maximize her tanking potential. Even Guard has a chance here. The fact that she's so fast already means you don't have to use Quick Riposte in B, which is the prime tanking skill to use.

All in all, I've considered something like this before but I don't know if it can realistically work since it demands so much more from Olwen than what her terrible BST can provide. It's nice to theorize about and I like the direction this is headed in. If we're theorycrafting, however, you have to use better more expensive skills.
Even if you're 100% intent on building her like this, she needs a great deal of investment. I don't think Fury is the best-in-slot, at all for example, there's so much competition for the A-slot here.

I hope this rambling helps.

Got a Olwen that I thought useless (+def -hp), but building her like this with an Owl tome might accually be a neat idea.