Best build for Oboro?

Well it's not that they're necessarily bad... it's just there are so many better options :P I saw a channel on YouTube using a hilarious carrot lance build that seems to work well for him if you want to check that out, though I'm assuming you don't have carrot lance. It's nice that you're having fun building your favorite characters, but as a perfectionist myself the fact that you 5*ed a unit with suboptimal IVs makes me want to tear my hair out >_> anyways, fury is always nice on any character and QR is nice for baiting, though you are vulnerable to ranged units. Overall, it's a good build, but silver lance is kind of boring and you might want to consider getting a killer lance to charge that bonfire faster (in be4 they introduce slaying lance)

Best build preferably on her is as follows;

Boon: +atk/+def -hp/-res
Weapon: Killer lance+
Assist: Reposition
Special: Bonfire
A skill: Distant Counter
B Skill: Quick Riposte 3
C Skill: Threaten Atk/Def 3

This is the best possibly build for Oboro but it doesn't mean you HAVE to go for it, this is what makes her effective but it's quite expensive to build so running Fury 3 makes her quite tanky and allows her to hit hard. Since you're already deciding to swap out her weapon I'd prefer going for Killer Lance+

Additionally if you manage to nab yourself a +spd Oboro, sticking Fury 3 on her puts her at 33 speed, which is enough to avoid most follow-ups from the average speed heroes.

If you wanted be a bit silly you could probably get some interesting results by giving her distant counter and Berkut's lance when he comes out. With a res boon and quick reposte I reckon she'd be a damn fine Mage killer, but again, don't take this one seriously.