Learn Portuguese (Portugal): Os óculos deles estão aqui.

In Portuguese, when you use a word like deles (their / of them) to show possession, you usually still need the definite article before the noun. Literally, the sentence is structured as "The glasses of them", which is why you need Os at the beginning.

Does deles change to match the word óculos or the people who own the glasses?

It matches the people who own the glasses! The word deles literally means "of them" (referring to a group of males or a mixed-gender group). Even if they owned a singular object, like a book (o livro deles), you would still use deles. This is different from words like meu or nosso, which must change to match the object being owned.

Why is it Os óculos deles instead of Os deles óculos?

Words like dele, dela, deles, and delas always come directly after the noun they belong to. Since you can think of them as meaning "of him", "of her", or "of them", placing them after the noun makes sense: Os óculos deles naturally flows as "The glasses of them".

Why do we use estão instead of são here?