Question, if words are plural, is that why it's comem? and O is Os?
Yes. We have plural of the (os/as). The verb changes depending on whether we're talking about I/you/he/she/it/we/you/they.
You can check out conjugations here: http://www.conjugacao.com.br/
Guris?? I dont remember where that came from
From the south region in Brazil.