This is the 25th most frequent Portuguese word.
os
"The" (masculine plural definite article).
'Os' functions as a masculine plural object pronoun, representing 'them' in the context of the sentence.
Eu os vi no parque ontem.
I saw them in the park yesterday.
'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.