Learn Portuguese (Portugal): Saímos agora ou ficamos atrasados.

Ou means "or". Just like in English, you can use it to connect two full actions to show a choice or a consequence. Here, it connects saímos agora (we leave now) and ficamos atrasados (we end up late).

The translation says "we are late", but why does it use ficamos instead of estamos?

The verb ficar often means "to become" or "to end up". While estamos atrasados means "we are currently late", ficamos atrasados means "we will become late" or "we will end up being late" as a result of not leaving now.

Where is the word for "we" in this sentence?

It is hidden in the verb endings! Both saímos (we leave) and ficamos (we become) end in -mos, which always signals "we" (nós). Because the verb endings make the person so clear, the actual word nós is usually left out.

Why does atrasados end in -os?

Atrasados is an adjective describing the hidden "we" (nós). Because "we" is plural, the adjective must also be plural, so we add the -s. The masculine plural -os is used if the group includes at least one male, or if the gender is unknown.