¡Hola, Christian!
Well "os" is a personal pronoun, same as: me, te, le...And it is used in many verbs, and always for the person "vosotros". For example:
- Verbos reflexivos y recíprocos (levantarse, peinarse, despertarse, esconderse, divertirse...)
Me llamo
Te llamas
Se llama
Nos llamamos
OS LLAMÁIS
Se llaman
Verbos como "gustar":
Me gusta
Te gusta
Le gusta
Nos gusta
OS GUSTA
Les gusta
Since in many Spanish speaking countries they don't use VOSOTROS but USTEDES, they don't use this "os" form either. That's why maybe you are not familiar with it. In Spain and other countries, we use it all the time.
Hope I could help :)
Marina N.