Learn Portuguese (Portugal): Oxalá a nossa estadia seja muito relaxante.

The word Oxalá expresses a strong hope or wish, similar to "let's hope" or "God willing". In Portuguese, expressing hopes, wishes, or desires triggers the subjunctive mood, because you are talking about something you want to happen, not a guaranteed fact. That is why we use seja (the present subjunctive of ser) instead of the indicative é.

Does Oxalá require the word que after it?

No, it does not. Unlike other phrases that express desire like espero que (I hope that), Oxalá connects directly to the rest of the sentence. You simply say Oxalá a nossa estadia without adding que in between.

Why do we use a form of ser (seja) instead of estar (esteja) if a stay is temporary?

Even though a stay (estadia) is temporary in time, we are describing the core nature or characteristic of the event itself. When you describe what an event, trip, or experience is fundamentally like (in this case, relaxing), you use ser.

I keep forgetting: why is there an a before nossa?