How do you think Twilight would feel about a nihilist or nihilism in general?

Most likely try to shove/force to solve it with friendship.

A person who has ocd like her?

She would probably understand it but not support it. I know I do...

Well, in my headcanon at least, Twilight probably has some philosophy based around harmony or friendship in a more fundamental sense. That said I think Twilight would initially be surprised and maybe a bit horrified by nihilism as Equestria is generally a very positive place. Then, and I think this would be very interesting to read about, I think Twilight would seriously question her own worldview and why she believes so strongly in friendship. I think ultimately, because of how integral harmony and friendship are to the fabric of reality in equestria, Twilight would reaffirm her beliefs. As to how Twilight would act towards the nihilist, I think she would try to understand them and show them how great friendship is.

Well that depends on what exactly is being nullified, no? You can have a nihilish disregarding religious principles and dogmas; and you can have someone else that disregards reality itself.

7504593
I think he was probably referring to materialist moral-antirealist nihilists

7504915

7504593
To be honest, I was actually mostly talking about Straw Nihilists. So with that said, what would she think about those kind of people?

7507674
Based on that trope I don't think my answer changes too much. I think maybe she would feel bad for them because, to her, life is full of so much meaning because of her friends. I think she would badly want to show him/her the friendship that makes her life so great. Also, assuming the nihilist is also a villain Twilight would grimly resolve to stop them if she cannot convert them.

I have to agree with Medofbr, her reaction would be that of pity.

Even before meeting the rest of the mane 6, Twilight valued knowledge so greatly than the mere act of learning about the world was enought to be worth living.