Learn Italian: Il personale, ossia gli assistenti, controlla l'ossigeno. - The personnel, that is to say the assistants, check the oxygen.

Ossia means "that is to say" or "in other words." It introduces a clarification of what was just said. In this sentence, it clarifies exactly who the personnel are. You can absolutely use cioè here instead; they are synonyms and work exactly the same way in this context.

Since ossia contains the word o (or), does it work like oppure?

No, they serve different purposes. You use oppure to offer a choice between two entirely different options. You use ossia to give a different name or a clearer explanation for the exact same thing. Here, the personnel and the assistants are the exact same people, so we use ossia.

Why is the verb controlla in the singular when the plural gli assistenti is right before it?

The main subject of the sentence is il personale, which is a singular noun. The phrase ossia gli assistenti is just an aside inserted between commas to clarify who the personnel are. The verb always agrees with the main subject, so it stays in the singular form controlla.

In English, "personnel" or "staff" often take a plural verb. Is il personale always singular in Italian?

Yes. Il personale is a collective noun, meaning it is a singular word that describes a group of people. Unlike in English, where we might say "the staff are," Italian strictly requires singular verbs for singular collective nouns.