This is the 25th most frequent Portuguese word.
os
"The" (masculine plural definite article).
'Os' is used here as the masculine, plural definite article, equivalent to 'the' in English, referring specifically to the male or mixed-gender group of students.
Os alunos chegaram cedo à aula.
The students arrived early to the class.
'Os' is used as a definite article, marking the specific books that were lost and now found.
Encontrei os meus livros perdidos.
I found my lost books.
'Os' functions as a masculine plural object pronoun, representing 'them' in the context of the sentence.