¿(v)os?

[ElDanés]

One of the first times I were having Spanish at school we were going through reflexive verbs like llamarse and levantarse, and the teacher asked if anyone knew how the reflexive pronouns looked like. I raised my hand and went through the forms, but he stopped me when I came to the second person plural form, where I had said, vos. He said it was incorrect, and that I should use os instead. I was kind of surprised as I had learned it as being vos, but after I had surfed the internet, I realized that it seems like both are equally used. So my question is simply whether one of them are more correct than the other, and if there is a difference between them grammatically, or maybe regionally?

[Tomisimo]

Vos and os are different. Os is the second person plural and vos is a second person singluar form. Vos can be thought of as an alternative to tú, and is used in some parts of Central America and Argentina.

[ElDanés]

The second person singular? As far as I know, the second person singular form is te.

me llamo
te llamas
se llama
nos llamamos
vos/os llamáis
se llaman

[Tomisimo]

me llamo
te llamas / vos llamás (or something like that)
se llama
nos llamamos
os llamáis
se llaman

I just did a bit of research on voseo (the vos form), and the reflexive pronoun is generally te, just like with tú.

So... Tú te llamas, Vos te llamás.

[ElDanés]

Well, I should simply use os and not vos?

[Rusty]

Correct, as far as a reflexive pronoun.

Vos, used only in certain countries, is a subject pronoun (replaces tú, and has a different verb conjugation than tú). It also appears where ti can be used.
Quiero darte a vos este libro.
Quiero ir con vos al mercado.
Esto es para vos.
¿Cómo (vos) te llamás?

[ElDanés]

Okay, thanks both of you!

Now I better start using os!

[Rusty]

Good idea, but only if you're in Spain. It isn't used elsewhere (excepto la poesía u otras obras literarias).

[ElDanés]

Oh? Now I'm confused. What's used elsewhere then?

(I tried re-reading the posts, but I couldn't find it mentioned - sorry, if I missed it anyway)

[Tomisimo]

The vosotros/vosotras forms (informal pluarl) are only used in Spain. In the Americas, they use Ustedes.